A cool plot twist would be if that the antagonist would be your persumetly dead brother . Which the main reason he goes evil is because of anger or hatred that you where not there to save him or that he was to weak to save your parents. Not sure if this is considerd to be to dark for the games liking or not. Or it could just simply be that your brother just want infamy and riches and you’re againts that idea or such. So he’d be your ally by the first 3 to 5 hours into the game but will turn bad, and you can see and tell why he becomes this bad antagonist whitin reasons and such and diffrent view over how that world should function.
Also how the boss will work would basicly Depend on what class you picked on in the end he’d be using similar moveset like you but only faster,stronger and more advance movset like you, and this will be a main 1v1 story quest, that you can only do in solo. Kinda how they did with Vergil in Devil may cry 3.