Bats would be so adorable. I’ve seen fan art of bat villagers and they look amazing. Also foxes would be great but we already have wolves and Redd is a fox so I think it would be difficult. I think turtles would be cute too! Like Kappn and Tortimer. The one species that isn’t in the game at all that I think should be there is another type of water animal like the octopi, so like a whale or sea slugs or something
Definitely more reptiles and amphibians, the balance is very mammal-heavy (though my island is usually crawling with cats and dogs, so...). Salamanders and lizards. Maybe cute bugs? Ladybugs and bumblebees? I mean, we can catch octopi and frogs, so isn't impossible.
Nerd note: I read this interesting article about the lack of biodiversity in emoji (link). I think the range of emoji has improved since 2018, but it was basically about raising cultural consciousness about different animals. I think AC is another opportunity for this, without being too heavy-handed about environmentalism.
Bats (NPCs and/or villagers), fox villagers, raccoon villagers, and more reptiles in general. Alligators don't cut it, and Nat and Flick are the only other reptiles.