Ahh this is hard; there's a lot of species I really like but that don't cover all the personality types, and that'd be a must-have for me. ;u; After some thinking, I'd probably go with squirrels and a woodland or forest theme—for them, I'd choose Filbert, Hazel, Marshal, Agent S, Tasha, Cally, Sally, Sheldon, Ricky, and Pecan. I think squirrels are really cute, and there's enough of them that I could cover all the personality types without having to double up on any of my New Leaf/New Horizons dreamies. Dogs are another good option for the same reasons, though, in which case I'd go with Portia, Cherry, Maddie, Goldie, Bones, Mac, Butch, Shep, Daisy, and Lucky.
I also really liked the idea of an all-frog island, where I think some kind of swamp theme would be really adorable. I'd have to double up on Lily though, who I already want on my New Horizons island, because I really wouldn't want Jambette, ahaha. If I went for frogs, my picks would be Diva, Gigi, Puddles, Henry, Croque, Raddle, Lily, Tad, Jeremiah, and Ribbot.
I like a lot of horse villagers too, and with those I'd probably go with Reneigh, Roscoe, Savannah, Victoria, Ed, Elmer, Buck, Cleo, Julian, and Winnie, although I already have Roscoe in New Leaf.
And if we can fudge things a little and call bears and cubs "one species", my choices for a bear island would be Beardo, Judy, Olive, Ursala, Murphy, Teddy, Nate, Pinky, June, and Grizzly.
...I'll stop now.