I like this idea, but this would mean that Guts' companions will have to change, not Guts

I think it's much more likely for Guts to eventually fall victim to the Beast. Remember the Beast's "last words" to Guts on the boat? The Beast is building up its strength while in captivity, ready to emerge and consume Guts again when he falls into despair. If that isn't blatant foreshadowing I don't know what is. I know some people don't like the idea of Guts slaughtering his companions(and I have mixed feelings about it myself), but I personally believe it would be a logical path for the narrative.

Not only it will increase her story role beyond being a plot device, but also act as a way for her to reward all the suffering he went through for her sake.

I don't think Casca really needs to reward him or pay him back. They love each other, and she would have done the same for him. The question of whether she would want to is a different matter, but I'm not certain it will be the first thing on her mind when she comes back. I could definitely see her wishing to help him tame the Beast, as she knows firsthand the negative effect it could potentially have on Guts, and the danger it poses to the people around him.