How about an enemy that is small like a snake that can slither along the floor and go through one block gaps? It'd make for interesting traps and stuff. It'd be nice to have enemies that can get you through a one block gap that you can shoot out of, like if you turn your back, suddenly, snakes! Just a though, doesn't really have to be snakes.
Heh. I'd totally make an Avian Temple with Snake-Pit traps. Mmm, I like that. Just imagine it, they fall down, and BOOM. Covered in Snakes, brah.