lol so this response is going to be kind of long because you made me want to talk about her story role and speculate a bit lol, so ty for sending this.
I think there’s a difference between having whatever Casca does when she gets her mind back further the relationship between Guts and Griffith, and having Casca function as solely a plot device for the sake of the Guts Griffith story.
By which I mean secondary characters should serve the central protagonist’s story, but they should do that while also having their own satisfying arcs that serve their own characters as well. So a well-written secondary character would have her own arc, her own issues, her own stuff to work through and her own reason and way of developing, and that could still shed light on the protagonist as a parallel, or foil, or simply by weaving their stories together and playing them off each other.
When Casca was an active character during the Golden Age, there were a lot of problems I had with her writing, but she did have her own story. It was a story about being obsessed with first one man and then another, and how much it sucks to be a woman surrounded by attempted rapists, and having emotional breakdowns, etc, so like, not a great story, but she had her own issues, she made mistakes based on those issues, she changed based on her experiences.
Eg when she and Guts slept together they were both using the other as a substitute for Griffith, so at least it wasn’t just Guts using Casca, they were using each other. (And I don’t mean using in a cruel way, just in a there’s-other-stuff-going-on-for-both-of-them-than-just-wanting-each-other kind of way.) That scene didn’t only further Guts’ internal story, it also furthered Casca’s. Ofc Guts’ story was furthered by working thru trauma and starting to recognize past mistakes while Casca’s was furthered by switching which dude she’s obsessed with, so like, still a shitty story for her, but c’est la berserk.
So yeah I don’t think her writing was that great during the Golden Age, but it cleared the bare minimum bar of giving her her own motivation and character arc at least, even though her own story was pretty weak compared her more blatant, main function of serving the relationship between Guts and Griffith.
Then after the Eclipse she became a complete plot device with absolutely no story of her own, only existing for Guts to play off of and project onto.
So I guess what I think is most likely to happen is that when Casca gets her mind back, she’s going to have her own motivations and goals again. She’s going to do something active, based on what she wants. But whatever it is she does is also going to further Guts and Griffith’s story, and lbr it’s still going to revolve around her relationships with the men. So hopefully she won’t be so much just a plot device, and her own choices, goals, actions, etc might even be stronger and more central to the plot than they were during the Golden Age, but Guts is still the protagonist so Casca’s story is sitll going to further his story and his relationship with Griffith.
My guess, based on where she was when she was traumatized to insanity, and where the story has gone since then, and where I think (hope lbr) the story goes, is that she’ll come back and be similar to where Guts was right after the Eclipse. In the last 200 or so chapters Guts grew, he worked at refocusing on his own emotional growth rather than revenge, he made friends, he chilled out, he’s in a much better place mentally than he was during the Black Swordsman arc.
But the story is still about the dark places trauma and desire for revenge take you. I think it would be interesting to shift the revenge theme to Casca. It would kick the plot into gear and make things happen because Casca would have a goal, this way we could bring Guts and Griffith’s narratives back together without having Guts’ development backslide into revenge obsession again, and it would make Casca an interesting foil to Guts – if she’s at the place he managed to work past, she’d be like a reflection of himself at his worst. Now Casca would be able to drive the plot, her goals would be the ones furthering the story, and Guts’ narrative would shift in focus from his own goals (revenge, fixing Casca) to reacting to Casca’s actions.
She would still serve the main story about Guts and Griffith by being the catalyst that brings them back together, by being a dark mirror to Guts, quite possibly by embodying the dark sexual undertone to revenge in a more blatant way than Guts did (bc lbr she’s always been the one to illuminate Guts’ desires by virtue of being a woman and making them hetero), and maybe by forcing Guts into making a choice between helping her and trying to stop her (either for her own sake or because he’s still ambivalent about killing Griffith or maybe both). But now she’d be serving the story by working towards her own goals based on her own experiences and her character, rather than by being a passive mindless object for Guts to interact with.
“What will she do if she does get her sanity back?” The fact that NeoGriffith instinctively saving her demonstrates a very strong disadvantage against her. The fact that the main characters all get to kill their own rapists/attempted rapists/abusers/etc. The way Guts decided he didn’t really have the right to avenge his comrades after abandoning the Band but you know who didn’t abandon the Band? The way revenge in Berserk isn’t always a bad thing, and it could be interesting to explore how it’s bad for Guts because he was basically using it as a form of self harm, but maybe for Casca it’s earned. The behelit which, if Casca is the one to use it, would open the door for more parallels between her and Griffith for Guts to play off of.
So I guess my overall answer to your question is that yeah, I think she’s still going to exist to further Guts’ story and relationship to Griffith, since that’s still the axel on which Berserk turns, but hopefully she’ll at least get to have some agency and motivation of her own while doing it, and if we’re really lucky her own internal story might be more important to the plot now than it was during the Golden Age.
But of course there’s always a horrifying chance that she’ll wake up and just be Guts’ love interest/narrative reward for moving beyond revenge, continuing to exist purely as an accessory to Guts rather than as herself, while something else moves the plot forward, but yk, prayer circle that that doesn’t happen. And like others have speculated with dread, there’s even a chance that she’ll join Griffith and make Guts return to rage and revenge in the worst possible way. I think it’s a tiny chance, like I really don’t expect that to happen, but you never know 
(also the whole revenge speculation is just my preference bc i want casca to have the chance to get angry about what happened to her, and it seems plausible, but there are probably other routes for her story to go that would bring back her agency and have her affect the plot in satisfying ways.)