It's actually something I've put a lot of thought into. With the introduction of Fantasia, I think it
might be possible to create a Berserk RPG set on the fringes of the world as we've been shown it, focusing on humans surviving by fighting through astral creatures. But in a way that (somehow) doesn't upset the predefined balance of power in the main series. However, it absolutely wouldn't make sense as a Souls-type game.
The major problem with that notion is that eventually you would expect the combat to scale up in difficulty, and thus the player character would encounter apostles. And what then...? The nameless hero takes down 5-10 apostles in one-on-one combat? It'd be apocryphal. Who is this character? And how would they fit in the scope of the Berserk world?
Games are about empowerment, but there's a hard wall in Berserk's universe about how much a human can accomplish. It's the basic premise of the series.
Now, if you make the protagonist an APOSTLE, suddenly things get interesting
but then, presuming the game is reflecting the Berserk world properly, battles would become too easy.