If K.K. Slider is a dog, and we have dog villagers, then there's hope for other NPC animals, I think (let's not forget Copper and Booker, either)!
Bats, yes. That'd be amazing.
Buuut, before they introduce new species, I believe they should make more variety of each already existing species first. We're lacking in the octopus department!
As some people have pointed out already I'd love to see the introduction of Sloth villagers, another one that I'd like to see is shark villagers, maybe some raccoons and turtles but not in the same style as Nook and Kapp'n's families.
This isn't exactly new but since the game already has bear cubs,why not more younger versions of existing species?Puppies,kittens like Katie,ducklings,piglets,wolf pups,etc....I'd like to see that.