Universal ability to control pets, similar or superior to specters: hold position, follow, attack, protect, etc.
Extend clan dojo with "useful" and/or "interactive" buildings, particularly: a forge such that players do not have to commute back and forth between forge on orbiter and then back to dojo in order to perform transactions. Similarly, perhaps, add all orbiter "interactive" constructs (relic refinement, pet incubator, etc.) to the dojo such that dojo can become a good place to hang around and not just a huge cosmetic trophy. Thankfully, at least, the dojo can be used to check arsenal and start missions from observatory, but perhaps extend the possibilities.
Same as the above, extend railjack such that railjack can become a valid destination to be at.
Show complete set of parts in fissures on mouse-rollover in order to be able to tell progression.
Ability to manage inventory / equipment in missions, a variation of: just observe, purchase slots, merge arcanes, etc.
Use collected void traces in fissures in order to refine relics, just as relics can be refined on relic selection but add collected void traces to the pool instead of requiring the mission to be remade (must be checked for being too overpowered in regards to looting and endless).
Extend specters such that they can be farmed similar to helping Clem instead of being mostly dropped from random rewards.
(Truly) merge matchmaking and chat servers such that parties can be still formed regardless of the time. Late UTC times frequently result in an empty recruiting channel for Europe, reducing gameplay time and effectively preventing players from playing the game. Allow cross-continent matchmaking, either automatically or selectively by user choice.
Add the ability to reduce graphics effects especially for long survival or defense - might help with health and safety and reduce graphics card load and improve the overall performance of parties (ie: game crashes without being able to reconnect). Effects in high level missions can stack a lot, filling the screen with just effects.
Radar overhaul: larger arrows for directions (particular, up and down) and better or faster path calculations. Many times the radar takes so much time to adjust that player intuition and previous knowledge of map tiles is used for pathfinding. Sometimes players end up getting lost or stuck since the radar shows an impossible path. Can be very confusing to new players. Similarly, after a long playtime within a mission, the overlay map seems to stop displaying or displays the exit destination poorly.
Allow zero-sum "relic sharing" within a mission: if one player is out of relics, another player can use / gift / give a relic for the player with a missing relic. An additional relic ends up consumed in the process such that it makes no difference whether relics are "shared" before a mission or during a mission.
Implement the exact same search engine and apply it to the star chart: players can then look up planets and missions by typing in the name without having to check individually if they do not know the destination. Similarly, but by extension, allow searching for resources via the search engine - perhaps, even, make the drop chances public such that looking them up outside the game is no longer necessary.
Allow combined-searches at the forge, with multiple criteria selection, ie: "in progress" and "melee". Perhaps tickboxes in the selection dropdown as an implementation method.
Allow in-game map beacon management, particularly, the ability to place permanent waypoints that will not be removed. This helps marking some locations permanently that are important but are not displayed by the rest of the interface during the mission.
Long standing request: universal vacuum / fetch - if the lack thereof is used as throttling, then at least increase the mix and match possibilities by introducing vacuum mods for primary, secondary, melee or perhaps arcanes.
[Unpopular toxicity source, contrary to just trolling chat with silly jokes]: eliminate, scale, change meaning of and / or fix mastery rank: unfortunately, MR is used as a veteran criteria but carries very little meaning in that sense given that all the end-game equipment, including Rubico can all be obtained even under MR15. After a certain MR, there is no more content locked and the only practical benefit is the one provided by the next rank, such as increased reactant storage which does not mean much in regards to "player skill" or "knowledge". Furthermore, MR is increased by leveling up items within the game, most of which are equipped by players, leveled up on Sedna / Hydron and then never used again thereby completely diluting the meaning of "mastery" rank. Unfortunately, MR, as opposed to play time and given that a lot of the necessary equipment can be bought with platinum, ends up increasing the toxicity of parties where a high MR, albeit excessive and redundant after a certain level, is just used as flaunting or flexing "skill" yet cannot be used as an accurate measure for neither skill nor does it indicate the lack of required equipment. It is entirely possible, given what MR implies, that high MR players do not know even the most basic mechanics of the game - similarly, it is possible for returning high MR players to not know about latest changes to game mechanics.
Additionally, mastery rank "tests" do not contribute to the player's knowledge in a timely fashion, for instance, the MR16 test is a "defense" mission and by MR16, any player would have ended up doing a whole load of "defense" missions.
Nekros nor Nekros Prime are mastery rank locked, most of the range mods can be obtained either by trading / purchasing or just even a little grinding yet Nekros Desecrate (with or without Despoil, depending on party dispositon) is an essential ability for any farming party. As such, a player with low MR, that maintains desecrate, learns not use shades is a player that any party will need regardless of the MR. The same can be applied to Nova and Nova Prime that is an essential frame for speeding up or slow down enemies - the Power Donation mod, not being MR throttled and can be obtained either by grinding reputation or purchasing the mod via trades.
Introduce, perhaps optional and/or rewards, bootcamp classes for every warframe acquired - similar to the small tutorial when starting a new game or playthrough. The bootcamp classes or tutorials could demonstrate to the player how abilities are used, why they are useful and would overall give even veteran players a sense of the game. Some warframe mechanics can be elusive even to the most veteran of players unless they are told or look them up elsewhere: khora has two cages, hydro can hold and load for more tentacles, tentacles scale in spread with range and in count with strength, nekros shades take up mob slots, nova can speed up or slow down the mobs on the map varying by strength and the range varies by duration instead of range, Khora's pet can heal other party members including the operator in arbitration, etc. This could be combined with augments and the changes applied to warframes when the augments are used.
Display AFK throttles visually: the AFK system has been "enhanced" such that talking or using abilities without moving a considerable amount will still mark a player as AFK even though that will not match the acronym AFK. Once a player is shadow-marked as AFK and if they are not aware, then the player might end up with fewer loot. Furthermore, being marked as AFK has various other implications such as not being able to receive energy. Displaying AFK visually and possibly with a toggle switch in the settings might make players aware and prevent confusion.
Warframe development over the past years has brought the addition of various areas such as Cetus, Fortuna, Railjack and Demios / Helmith. Unfortunately all these additions have a separate set of resources that are rarely found in other parts of the game. By consequence, the new regions ends up being a sub-game in themselves with little seeping of consequence to the rest of the game. For instance, playing railjack will yield rewards that are mostly just relevant to railjack itself; aside from a few resources that can be used to build some non-critical new weapons introduced. In effect, one ends up doing Fortuna, for the sake of Fortuna, Railjack, for the sake of Railjack and with an arguable exception being Cetus that pertains to much more to the operator that does have an effect on the rest of the game. Sprinkling or spreading resource drops allover the entire game might yield a better distribution of gameplay for all areas of the game instead of ending up with a disparity between low and densely populated player areas.
Touching on the MR case, there is a major scaling issue with weapons that regrettably make a mockery of the artistic weapon designs: even though a lot of weapon designs are aesthetically pleasing, many of them are considered, by the players themselves, to be "MR fodder" - meaning that they are just used to bump MR and then discarded. This is due to well-known meta-configurations such that specific weapons are ultimately superior overall in the entire game. Perhaps a single most-notable exception would be a statstick Khora, where any melee weapon can be picked in order to deal more damage since the weapon chosen has no effect on the resulting stats or damage. However, for the most part, only a few weapons matter and entire sub-classes of weapons, ie: fist weapons, are entirely "MR fodder". For a game that has three separate weapon types with various subclasses (fist, whips, swords, etc.) the fault can be seen as pretty severe.
Some key-weapons are more powerful than others regardless of MR and even regardless of prime status: an Ignis or Atterax will overall end up being more powerful, easier to obtain and more powerful than even a prime weapon. Why farm Paris Prime with void fissures when you can just use the Ignis (Wraith)? Why bother with Braton Prime when a Hek with Scattered Justice will always end up being better (in terms of overall damage)?
Similar to the weapon scaling issue, due to various adjustments, either nerfs or buffs, as well as the gameplay itself, only a select few of warframes are sought after, with only quite a little handful of frames being crucial or indispensable. The result is that, by contrast, a pilfering or whipclaw Khora ends up having many more uses than an Oberon - the effect is that a player with a low-sought-after frame will end up not even being able to attach themselves to a party. The issue is majorly relevant due to the overwhelming amount of frames that one can pick from, such that the sense of identity (implying, skill) between the player and the warframe is lost, the frames becoming essentially "tools" that resolve jobs and no more than that.
The starchart has some reward distribution anomalies, with well-known spots for certain resources. For instance, Sedna / Hydro is perpetually being played due to being a good well-known mission to farm affinity. Such anomalies lead to a disproportionate player distribution across the game as well as not really being in-line or backed up by any sort of lore: there is nothing special about Sedna / Hydron, it's just another mission on a planet where a lot of players end up in order to level up weapons. If that is the case, why not make affinity farm "special" and dedicate a region of the game therefore - otherwise Sedna / Hydron remains an in-game "trader secret" for no particular reason.
More to "in-game trader secrets", there may be a failure to advertise some opportunities, such as selling credit boosters. For instance, just two rounds of high index with a credit booster will result in 1 million credits which is considerably faster than running around the star chart. There is no skill required except finding out that high index can easily generate credit rewards. Conversely, most index parties are formed just for the purpose of accumulating credits or, perhaps, yet just once, getting the Prodman sigil. Perhaps, index could be extended in order to fulfill more roles than just what players are currently using the index for.
Aside from Maro's bazaar, used for trading, relays tend to remain empty most of the time, perhaps filling sometimes due to Baro's arrival. It would be nice to be able to have more activities to do at relays except visiting for reputation rewards and perhaps simulacrum.
Mini-game tournaments (linking with the former, at a pub within a relay?).
Reinstatement of raids, or, at the very least, allow "large" parties for certain game content.
Similar to raids, due to only 4 party members being the maximum and since a lot of the content can be obtained without a party (or using PUGs), the game tends to shed its "multiplayer" aspect where players end up mostly on their own (ie: completing star chart missions).
Quests are too few, the lore is abundant yet very elaborate or hard to follow where palyers end up mainly completing quests in order to just unlock game-content. Once quests are completed, they are forgotten and discarded and it is pointless to do them again, except, perhaps with a friend. Recruiting or finding a party to accomplish a quest such as The Jordas Precept is a painful experience. Why not introduce a cut-scene after the completion of each relay, after moving to another planet, that explains the story so far as the players complete the star chart? That way, the lore would be seamlessly exposed as players complete the game. The idea would follow NCSoft's GuildWars 1 progression style where every completed key-area is followed by a small movie that explains the story and gives meaning to what the players have been doing in context of the lore.
There is little progression indication; you can do whatever, whenever and wherever - this tends to be a good feature of warframe but optionally, it would be nice to at least have some indicators showing the order of completion such that a player can chose to follow the lore as it is meant to be followed. Otherwise, players can end up in ironic situations where, for instance, the Limbo quest tells the player about "a mysterious new warframe [that nobody heard about]" whilst the player has Limbo Prime equipped.
Following redundant weapons and low sought-after warframes, the addition of the hoverboard in Fortuna seems extremely redundant due to the archwing being usable in the very same areas that provides massively more movement speed than the hoverboard by contrast - anecdotally, why go to war on a hoverboard when you can fly around in a plane? Why not give more meaning to the hoverboard by reusing already existing in-game assets rather than having the entire hoverboard design ultimately end up as "MR fodder"?
The introduction of the loot Smeeta buff, along with Fetch (as the pet-equivalent of Vacuum) make players tend to use Smeeta mostly where sentinels have ended up being ignored for the most part. A very "interesting" as well as useful sentinel would now be "Helios" that manages to accumulate Simaris standing by scanning mobs passively. Historically, Carrier used to be the only one to benefit from Vacuum but now all sentinels and pets have some form of Vacuum. Perhaps introduce reputation mods similar to "Helios" for pets as well? Or add loot buffs to some sentinels? Either way this follows the line of reasoning where optimal configurations ("metas") are well-known and the preferred way of playing the game making other configurations seem redundant.
The universal ability to skip any sort of cutscene.
Optionally, disassemble weapons or frames, perhaps with certain resource refund probabilities, if throttling is desired, instead of selling the weapons or frames for credits.
An omission perhaps, but the inventory screen only displays weapons without breaking down into primary, secondary and melee.
The ability to give a name to any and all weapons - similar to gilding. "Funny names" aside, there is some utilitarian purpose to naming, similar to pet naming, where a "Smeeta" could carry a name containing "Smeeta" in order to announce the type of pet being used.
Include warframe mod-slots polarity as part of loadouts allowing users to swap between builds. If throttling is desirable, then ranking up the frame for every loadout is feasible.
Since each tileset is randomly generated in Warframe, it would be nice to create a hash based on the result of the randomly generated map and expose the hash to the players. That way, players would not have to reset the mission in order to obtain a desired tileset disposition. Players would only have to note the hash and then know whether the game has generated the map tileset that they need without having to restart the game.