Well Griffith trying to ascend to the throne thru military feats and heroism would ruin Miura’s ye olde sexism vibe, so he’d either have to boil Griffith’s whole narrative down to just seducing a prince thereby changing the entire plot, or go full (less campy) xena-esque historical fantasy where misogyny just isn’t really a problem. Which would actually be awesome, and if Guts was a woman too that would be the ideal story lol.

Also obviously Guts/Griffith would be considered canon. Like even if it was the exact same level of subtext as the actual story, Berserk would be known first and foremost as a tragic dark romance, and the fact that it’s full of blood and action would be considered secondary. No one in the world would read it and think Guts and Griffith were just platonic friends. I mean, I guess if I were reading it I’d deliberately choose to see them as platonic friends lol, but even I’d know they’re supposed to be in love.

I mean like, if this scene was hetero it would instantly seal the deal on their inevitable romance lbr, you’d be called delusional if you didn’t acknowledge it:

And it’s just one moment out of so many lol.

Bringing Casca into the picture, Miura would go just as hard or harder on the Guts vs Casca as romantic rivals vibe. They probably wouldn’t have a bonding moment followed by burgeoning friendship tho, and obviously they wouldn’t hook up. I figure they’d remain standoffish and surly until the Eclipse, at which point something else would drive Casca insane and Guts could still eventually grow as a person by choosing to take him to Elfhelm bc he’s all that remains of the Hawks or w/e. Or more likely he’d die and something else would drive Guts’ Millenium Falcon narrative.

I don’t think there’d be much subtext between Guts and Casca in this theoretical version of Berserk, bc if Casca can’t be Guts’ actual love interest and no homo bridge between Guts and Griffith, what would his role in the story even be, other than the dude who brings Guts’ insecurities out (ie Casca’s pre-romance role). The story’s all about Guts and Griffith, making it het just cuts out the middle man.

On the other hand, if Guts and Casca did still eventually become friends and eventually Casca became the symbol of Guts’ humanity and way of distracting himself from Griffith, I might ship it lol. I can’t lie, I’d actually hate griffguts if it was het, especially if Griffith was a woman, and I’d be searching for an alternative. The narrative about destroying yourself and your ambition because you fell in love would be extremely not my thing if it was a woman falling for a dude, Griffith sacrificing Guts for her dream in this case would probably make me cheer lol, and I’d be first in line to suggest that NeoGriffith’s feelings aren’t real.

So idk I guess my answer to your question is that it would probably still be love triangle-y, but instead of Guts and Casca getting together the focus would stay on Guts and Griffith, probably with Casca as a more minor character. And the plot wouldn’t necessarily have to change, but the setting would to an extent, if we accept that a woman could become a war hero as part of a rise to power.

Also a lot of details would almost certainly change, like Casca’s backstory of being rescued by Griffith and hero worshipping her. I imagine Miura would be more likely to emphasize the protective aspect, and have Casca fall in love after seeing Griffith vulnerable, after joining the Hawks just because. Griffith saving Guts and Guts feeling like he’s found his home could be the same, but if Griffith is a woman I wonder if Guts’ backstory would change a bit to emphasize his mother – maybe instead of dying she’d take off. Yk, that kind of change.

My preference would be to change very little just on principle tbh, but I’m trying to think of what Miura would do lol. Anyway idk, this is getting long so I’ll end my answer here.