Eva's like my favourite thing, got all the shit I love: Alien hypertech, human hybrids, grand conspiracies, magic and tech being interchangable, ramiel, girls that smell.
but how to make it
Gameable
playing as the Fucked Up people getting sent to fight the Fucked Up Monsters is neat but it's all controlled by higher ups. Like, ye, a class design would mean that sure, your berserker mech is indeed a mech that goes sicko mode more often than not. But you're never gonna have the chance to like seek out an S2 engine.
One idea is that players have two characters. One's doing dirtywork and is an emotional wreak, the other is doing dirtywork and an emotional wreak but ALSO, has a measure of influence.
So like, if you're Shinji, you also play Ritsuka.
Something something, you need at least one degree of separation between your characters. So you can't be Shinji AND Misato, or Gendo, but you can be Kaji. To prevent Talking to Yourself moments.
The Angel set up would need some fiddling with. What if the other Eva programs... worked? Competing Evas and design philosophies. American Evas prioritizing ranged combat and slap their pilots into learning how to neutralize AT fields at range real well.
something something, Pieces of Lilith/Adam scattered about. Everyone's trying to reassemble Adam/Lilith. Uh Oh, an American Jet just happened to drop this dummy plugged, berserking Eva RIGHT ON THE GEOFRONT. Oops. Whoopsy. We're sorry. We'll be taking this black box, step back, I have a gun, you can't stop me.
Hey asuka, pop training exercise. If you can take out that [DRONE] then you'll be known as the bestest pilot ever, forever. You'll get to tell shinji this.
could be fun. Reduces the "if we lose once its fucking over" of EVA which blunts the omega-truama from "the world ends" to "you made a WMD because your ass couldn't stop it, get in this borrowed EVA, yes its going to feel weird."
So, to maximise agency in an Eva setup:
Players have two chars, pilot and a NERVous wreak.
These two chars have one degree of seperation from eachother. [maybe someone like misato is like, GM controlled. Caretaker.]
NERVous wreaks harbor some actionable Secret in the agency. [Ritsuka knew Rei was a clone. Gendo knew The Plan. Kaji knew that NERV wasn't what they say they were.] Actionable meaning some leverage can be applied. Letting a pilot know, telling the world/country and getting Spec Ops on your ass, hijacking the operation from Those Above.
Failing a combat is possible but the world doesn't end. maybe some Contingency needs to be popped [Lance of Longinus deployed, need to go in debt to another Superpower], or, a new, worse combat pops up where failure here CAN result in end of the world.
big, big paragraphs in GM section about if the world ends. god, fuckin' imagine, world ending being its own state of play. semi-domain play with restoration being the goal. too ambitious. maybe. maaaybe.