this is kind of a big request bc it'd make the programmers have to do an entirely new mechanic, but i love the idea of dolphin / shark / seahorse / other aquatic villagers! we already have the octopi, but they move around on land?? i think with these aquatic villagers, they would plot their houses in the ocean! and the human characters would automatically get a wetsuit (and have it easily equippable / not taking up an inventory space all the time) so they can swim to visit their water-dwelling villagers!
this is different from a villager-species idea, but going kind of along w the houses in water idea, i'd love it in future animal crossing games if we could put plants in water like lily pads and pretty looking floating algae and hyacinths :')