What would be nice is just let all characters be playable in S1 story mode. If a non S1 character beats Fulgore or Shago , give them the Aria ending. Simple yet effective while utilizing the tools in game, no need to create any new story boards ect.
Of course Id rather have story boards but a sacrifice is fine.
Or use Survival engine but remove the Survival health restriction. Place Gargos and Eyedol at the end.