Maybe an aquatic theme, deep sea or something like that. Sharks, Octopus, even bring it up higher and do a barrier reef mode or something like that with a six ball multiball to simulate the fish on the barrier reef. I could see some interesting toys and modes...maybe even throw in a shark cage with a diver, have to save the diver from the sharks or something.
I think it could work with some thought.
I thought Spelunking was a good idea too, wide open theme with cave collapse modes, stalactite multiball etc.