How about a Tarot-themed or Card-themed frame?
There's a few ideas I've thought of with this:
1) Shuffle & Draw. This version uses an ability that shuffles the deck, then draws 3 cards. The drawn cards provide buffs for the player and their allies, and remain around the frame (perhaps circling their hand). Each suit and unique card correlates with a different buff and ability, while the number on a suited card determines the cost and power of the ability. 2, 3, and 4 activate the card's use ability. Pressing and holding 2, 3, or 4 for a couple seconds will discard that card, and a new one will be drawn. It could have a limiter in place to prevent 2 of the same suit from being up at once.
- The good in this is that you have a frame with more variability and dynamic opportunities. It may take some discarding, but a player can eventually lock in a set of cards that they like, and will keep them til the end of the mission or a death and revive.
- The bad is that the lack of consistency means it is tougher to rely on any specific abilities, and allies have less of a specific idea what the frame will actually be providing. I expect players will spend time drawing til they get what they want.
2) Kill to Draw. An alternative is that the creatures they defeat determine the cards they draw, so players can discard and selectively get a card by killing a certain type of enemy. This might mean that certain buffs and debuffs only be possible when certain factions are being fought, and I don't think that is a problem, because it means the player will be getting abilities that specifically help with defeating their current opponents. Unique cards could be earned for bosses and special enemies, while suited cards could start at 1 and build up to 5--or whatever the upper-limit is--for repeat kills of the unit type which provides that card. This doesn't require ability 1 to be "draw" so this provides 4 abilities, but the hold ability number to discard mechanic can remain.
- The good in this is that there's variance that is specific to the enemies being fought, so there's a bit of variance, but it should make them more adapted to the situation.
- Despite that, the bad of not having a predictable set of abilities to use and buffs to provide can still make them awkward to play alongside.
3) Choose Your Hand. The player CHOOSES the cards. Holding the ability slot brings up a gear/emote selector with the cars which can be used at that level, and the player assigns them as needed.
- The good is that it creates a very dynamic and adaptable frame, which can change as needed to match the situation.
- Still, the bad is that you have no idea what the Tarot frame player is going to use unless they tell you.
As a bonus, Prex could add to the frame's ability set, expanding beyond the basic suits. This creates a frame with an alternate, but hopefully meaningful, progression system. Or not, it could use the images from the Prex cards, though!
Just as much as their gameplay capabilities, the emotes and roleplay elements should be pretty nice, even being able to do a Warframe Tarot reading in common areas or even mid-mission while you're waiting for the party slowpoke to make it to the boss battle trigger. Community artists could even make cool art with them, or DE could partner to have physical decks made and provided to donors as part of a charity event.