What if I, as a Deva, decided to do evil? There should be some way we could interact with humans, at least negatively. They way I understood devas, they fall down into lower reincarnations because everything is so easy for them... Also, can we die of natural reasons? It'd be cool if we caused havoc among humans and they had to, somehow, get rid of us or get protected. Some kind of shamanic humans who could defend other humans by meditating AND doing other things at the same time to be able to fight devas.
Pretas & lost souls could get some kind of bonus by attacking humans so they do it more often and we devas could try and defeat lost souls/pretas in exchange for good karma OR harass humans for bad karma... dunno. Would very much enhance the game, IMO.