I have no interest in having an island full of all of the same species. One of my towns in New Leaf is an all-bird species town, which has at least one representative of all of the different avian species in the game: Bird, Chicken, Duck, Eagle, Ostrich, and Penguin. I enjoyed doing that, but part of that is because some of my favorite animals are birds, like crows, hawks, ravens, penguins, and owls. Since there are so many different types of birds in real life, and since there are multiple bird species in Animal Crossing itself with different designs and personalities, it was more interesting to me than just picking cats or dogs or something.
I like coming up with themes to base my towns/island around in Animal Crossing games, and my New Horizons island's theme has already been long decided and built around; world history and cultures.
There is actually some unintentional overlap occurring on my island, though. I have Klaus representing Ancient Rome and Pekoe representing China, and while Bears and Cubs are classified as different species in Animal Crossing, they're both technically bears. I also have Ankha representing Ancient Egypt and Kabuki representing Japan, and they're both cats. Finally, Diva is the only Sisterly who I hadn't used yet that interested me, while Ribbot is my Jock, so another accidental pair of Frogs. I'm considering moving Diva out in the future though, after I get all of her Reactions, but I'm not entirely sure that I want to do so because then I'd be replacing her with a third Cranky villager and that seems a bit redundant.