So after a long talk with
@Turing Decidable I have thought of a fusion fic for Eva and Fate. It would basically be the Eva cast transposed into the Nasuverse with a holy grail war about to start. Gendo Ikari is a mage who wants to use the grail to bring back Yui whose soul was ripped apart in a root experiment. To mix Eva and fate the servants are all humanized versions of the angels from eva (well except the ones who are already human shaped). Shinji is forced to join the war after summoning Tabris by accident as his servant. Asuka is a hot blooded exchange student from Europe who is secretly a mage herself and the chosen representative of a number of mage families. Touji is another accidental master who summons Bardiel (Assassin) after being told he can use the grail to heal his dying sister. I have not figured out everything but I think I have a foundation for something cool.
The angels/servants and their roles are as follows:
Zeruel/ berserker
Bardiel/ Assassin
Ramiel/ Archer
Areal/ Caster
Iruel/Rider
Lilith/ Saber
Tabris/ Lancer
Eh, I feel that the idea of Angels replacing Servants is rather uninspired. Kaworu works, sort of, but the rest lack any personality or ideology of their own, which drastically reduces their narrative role. I mean, you could go with OCs and give them voice, but at this point it's easier to go with mythological heroes and maybe give them some kind of amalgamation between their canon cool stuff and Angel abilities as NPs.
Reimagining Eva characters as natives of Nasuverse has some potential, though I feel you don't make full use of possibilities here.
Gendo actually works better not as an established magus but as a husband of actual magus, Yui, who picked up some tricks (making him a first gen magus) but whose true strength lies in politics and making contacts rather than craft. This way, he
needs Shinji to participate since he's incapable of supporting a Servant with mana himself, which would create a dynamic between them similar to Eva but probably with Gendo being more involved by necessity since he would need Shinji at the very end as well.
Asuka works well enough with her canon backstory slightly adjusted for circumstances (mother going mad from some experiment, suicide, dolls, adoption, becoming a star genius, etc.), though I would make participation in the ritual her own choice not backed by any family, her way to prove her worth to the world. The Holy Grail War is considered the culmination of modern magecraft, so winning it would be a sure sign that her existence matters.
(For bonus fun, I'd make her family magecraft dollmaking similar to what Touko does.)
Toji's involvement feels weird. I think he works better as someone otherwise uninvolved who gets caught up by events, like maybe Caster decides to use him as an ingredient in ritual or something.
There is a matter of Rei to consider. In this context, she's obviously a homunculus and probably the vessel for the Lesser Grail, designed to unravel after consuming the souls of Servants and give rise to the miracle. I kinda like the idea of her being a second vessel, similar to Sakura, something designed to steal the miracle and subvert the normal process of the war. Gendo is behind it, of course, but his involvement is second-hand, the person responsible for her creation is someone else skilled in alchemy.