The most common complaint I hear from friends starting warframe is the warframe slot system.
New players get 3 warframe slots. And if they progress through the nightwave they get one more. So if a new player doesn't skip any of the warframe farms they would have their starter, koumei, rhino, and excalibur/xaku/mag. A new player would have maxed out very quickly, even if they somehow managed to reach rank 20 in the nightwave before reaching mars. Using the starting platinum new players can increase this limit to 5 or 6 slots, still not a whole lot.
A few of my friends have actually dropped the game at varying stages of completion as a result of the warframe slot system, they like the warframes they have and don't like how limited free warframe slots are. A few have engaged with trading to get their slots, and one has resorted to buying platinum.
Unfortunately warframe slots are likely a pretty big source of income for DE like forma. But I have a potential solution that shouldn't reduce that income too much. Warframe cryostasis. Such a system would allow a player to remove say, excalibur from their inventory so they can claim a new warframe without having to sell excalibur or feed him to the wall. Then, if they decide they want to play excalibur again they can store another warframe and begin "thawing" the same excalibur this hypothetical player invest 4 forma into.
There are 3 key limitations that should prevent this system from eating up too much revenue.
A warframe cannot be thawed without an available slot. Thawing a warframe would add them back to your inventory but they cannot be used until fully thawed.
Each warframe would take a minimum of 24 hours to thaw.
Prime warframes cannot be put into cryostasis. This would require players obtain a minimum of (currently) 49 warframe slots in order to collect every prime.
(Optional limitation) The use of Orokin Reactors and Forma could increase the time necessary to thaw or entirely prevent that warframe from going into cryostasis to further reduce this systems impact on income.
I believe a system like this could help to alleviate this pain point for newer players without being so useful that people just stop buying warframe slots.