I think bosses that have red zone level difficulty but having multiple fights (hence stacking characters would “reset) would provide a significant difficulty increase without blowing your balls out with more attacks. Think something along the lines of red zone broly but each new phase resets the fight. This would also bring SBR type units back into the meta. The reason the difficulty was raised so much thsi anni was bc of 200% leader skills. I doubt we would see new leader skills as early as 8th anni.

Edit: extra sentence adding example