spirit form
Well, at least it wasn't a big spoiler. I was fairly sure I'd need a different form to get through vents anyway, the only difference is I now know the name of it.
Another idea I had was, since the game is based around singing, why not have players singing together? Give combat bonuses when two people sing together, and even more if there are three, and so on. The more people there were the harder it would be to sing the same song at the same time - it'd get very hard very quickly, so after you had five or so people you'd be able to bring down a whole screenfull of monsters with a single song.