Only trouble with splitting the bosses down different paths, if you want to call it trouble, is that it'd be like letting you practice the last part of a boss fight without clearing the earlier ones every pull, to some extent. Not saying they necessarily balance around that, but that may be a consideration of theirs when designing em.

But, since we're just kinda spitballing here, how about do some Megaman shit and have the boss' stages (trash and mechanics) change based on which ones you beat first.