In short story and design:
Frame based around a person being weak and vulnerable normally, that to gain power needs to react accordingly to the enemies attacks and movement, type of enemy and type of damage being received.
Abilities/Design Philosophy:
A) REWARD PREDICTING ENEMIES MOVEMENT
instead of the usual ability that explodes everything at the press of a button, if it has one, either it explodes much later and needs enemies inside it to trigger OR it acts as a limited deployable that damages enemies most effective in a chickepoint that gets more powerful the more it stays active
B) REWARD CREATIVE USE OF THE ENVIRONMENT AND MOVEMENT
such a weak frame is perfect for giving it wacky mobility options. Like a Nova ( but on dr*gs xD) This frame can create many construct that alter how it and teammates navigate the map, giving options in combat and provode cover. It must feel as if you were playing Fortnite xD Examples are walls, pillars with watchtowers, portals (but with quick rail animation, "void rails???"
C) ENEMY POSSESSION
I know it can be a challange from a programming standpoint, and requires much of the game code that I have never seen to be structured in a way such as to allow such feature, BUT ...
it is about time that Warframe implements this! xD It can be specific enemies only, or even simpler, each faction has a pre-made enemy thypes this frame can Transform into.
The controlls and abilities would be a challange, as if Warframe movement is maintained, the difference gets lost and the mode looses its uniqueness ... on the other hand, if we were to play as Veso or Teshin or Kahl, they would need to be SUPER POWERFUL AND SUPER SHORT LIVED, as their limited mobility risks hurting the flow of the gane players are used to.
D) TIMED PARRY MECHANIC
This would greatly increase engagement from the player that would otherwise just spam attacks. Works with both melee and ranged attacks.
E) DAMAGE-INDIPENDENT BUT LIMITED SHIELD
This Ability would create a frontal shield ( that in reality covers all 360 degrees ) that can absorb INFINITE DAMAGE ... BUT, it has a limited use timer, it drains quickly and takes 2-3 times the amount to reacharge. Can be charged by kills or other tennos using your mobility devices, portals or standing on a platform you created.
F) MORALE MECHANIC
If this frame successfully takes out a VIP/Heavy unit while other troops are present, it fills a morale/determination meter if it has one like ember, or. erby enemies get a debuff, or even go go bezerk shooting at their companions.
Thematically, it is neat as one thing is loosing to a hulking void-powered God ... another one is seeing your Eximus Heavy gunner boss get owned by V from Devil May Cry 5 as a quick example 🤣
( these are the rough guidelines ... I may embelish this .... If I really wanna make this cursed, I'll even draw him/her/they/it ... )