So many!
Right now I'm working on a "whac-a-mole" concept within the polygon. Letting players run around from place to place to smash things with their hands sounds like a good idea to me. I have countless amount of idea for arcade games that can be ported to VR.
Otherwise I want to explore more strategy-oriented ideas (tower defense is my top candidate), and maybe some action game (the thing you said about the door made me think - you could divide the polygon to two halves, each is a different world, and you need to move between them all the time). Anyway, there's SO much potential there IMO