Considering the upcoming new enemies, I think it would be cool to implement a ship landing on the lava island, with new enemies arriving on a saucer and NPCs after completing the storyline, giving access to a new space-themed map with reduced gravity, along with new enemies.
Regarding gameplay, I'd add more Jokers so you can regenerate a teammate's HP with a hit from a ghost soul, and add the ability to glide or perform additional jumps (since you can't climb ladders or ropes).