Idk if the stipulations are that she has to survive with some trauma represented post-Eclipse, then I guess what I'd do is give her the same trauma as Guts, ie watching all her friends suffer and die horribly and being betrayed and maybe maimed in some way (not rape), and then give her a secondary narrative where the way she deals with the trauma is paralleled and contrasted to Guts.

Like maybe she gets the behelit and he gets the armour, or vice versa if the Black Swordsman arc is already written lol. And they have to go their separate ways because worldbuilding establishes that having two branded people in the same place for any amount of time causes astral plane chaos, but also neither of them wants to deal with the other anyway so they're fine with that.

I would dislike it if Casca got the wholesome healing narrative and Guts got the violence and rage narrative though :/ Maybe they demonstrate opposite spectrums of negative reactions to trauma - like Guts gets obsessive and violent and goes on a revenge rampage, while Casca becomes cold and aloof and numb? Idk what her narrative could be about in terms of plot though. Maybe she's the one who meets Farnese and has kinky sexual tension with her, and Guts ends up at the Tower of Conviction through his monster-hunting, and then a version of the Conviction arc and Griffith's resurrection happens while they're in the same area? Then some other plot happens to distract them from Griffith for a while?

I think there's a way the Eclipse rape could even be incorporated if it was depicted in a much less misogynist way and her trauma after was focused on, rather than just transforming her into a personality-less plot device. But in terms of personal preference, I'm more interested in a story where she isn't raped, so yeah.

Idk. Miura kinda wrote himself into a corner by not including Casca during the Black Swordsman arc but then having her survive the Eclipse, so I can see why he felt he had to find some way to write her out. I think having her die a cool, non-sexual death with the rest of the Hawks would've been more fitting and a lot less misogynist overall lol, but failing that then imo she should've stayed a character and gotten a chunk of the story focused on her own post-Eclipse arc.

Alternately maybe she could just go off on her own without the narrative following her side of the story for a while, maybe becoming a tertiary character who occasionally returns to contribute something and justify her improbable survival narratively, a la Rickert but with more plot impact? It would probably be less clunky and more streamlined which I prefer when it comes to plotting, but could also easily feel cheap and still somewhat misogynist if only Guts' trauma gets focused on, so idk.

I think it's a tricky situation to write any way you slice it really.