A bit of an analysis here.
Animal Crossing villagers tend to animal species that don't overlap with the species of the NPCs. This I think is a design aspect to help make the likes of Tom Nook, Redd and the like more unique to your town and help make them stand out among the residents. Even when they are the same species, like the dog npcs, they are most notable different breeds with unique designs to convey this, whilst dog villagers are a generic sporting/non-sporting breed of dog.
The only one's breaking this rule are Rover/Blanca and Porter respectively using the exact same model for cat villagers. But for design reasons I can see why they would be. Rover being somewhat of a bridge between special NPC and just villager designs in general as he is the first character met, also serving as an intro to the game's character designs in general. Porter came before the monkey's were villagers and their debut game doesn't have Porter in it, so I think its a bit of a reusing of current assets to create more type scenario.
What I'm trying to say here, is that it is unlikely to think we'll get foxes and such when it could possibly take away the uniqueness of the NPCs.
As for villager species in general, if they ever do add more in, I think we need to take into account that they would want to introduce new species that would visually stand out among the other current villagers.
Meaning that animal species that may visually look similar to current species may just be added in to become unique villagers among an established species of villager. For instance Boone the Gorilla; mandrills are not gorillas, they are Old-World Monkeys, but his design works for the build of those villagers. Same with others such as a Kyle, Flora, Anabelle, Phil and so on.
So when it comes down to Red Pandas, I think its more likely to see a cub villager that looks like a red panda with a tail. Same could go for Platypus villagers being unique duck villagers.
With how Bull and Cow villagers exist, I would be for seeing a Lioness species that could also be used to have cougar, panther and cheetah designs introduced into the game as well.
With foxes its a hard one, since the large, roughness of the wolf villager design don't really suit it and using Redd's design would make him lose his uniqueness. So if we ever did get NPC species for villagers. They'd probably be designed to stand out and not be exactly the same species.
So for foxes, they'd be designed to be more Fennec Fox-esque or possibly just a smaller version of the wolf villager, in a similar vein of bear and cub villagers. Llamas could be introduced to serve the alpaca want, but they would be designed differently to stand out from the Re-Tail couple. Owls would probably be a Barn Owl or similar species in design to stand out from Blather's Great Horned Owl design. So it becomes this concept of people wanting to have multiple Blathers/Celeste, Reese/Cyrus running around in their town, not realizing partially that if we do ever get these species, they probably won't look as close to these unique characters as some may want. Same with lizards as villager species, they'd probably be a basic looking reptile, somewhere between alligator and frog villagers to allow Nat and Flick to stand out as Chameleons. Maybe something closer to Frillard. Turtles/Tortoises are a hard one to keep have without lessening the uniqueness of Kapp'n and Tortimer.
For unique species that could work, you also need to see if they could work properly in the design element for villagers. Seals for instance, either though we have Phineas the Sea Lion or Wendell the Walrus, sea lions/walruses are built different to seals. Actually having a build and separated hind-legs that allow them to walk on land and sit their bodies upwards. So when we see him walking it makes sense, but seals don't have that skeletal build and their hind legs are joined as flippers, hence when we seals on land, they are sliding on the ground or bouncing their blubbery bodies along. If they were villagers, they would be this weird balancing act for walking around and not make much sense. Aquatic villagers keeping to the ocean would then just be a problem all on their own; the move-in process and interacting with them entirely. So seals, manatees and duogongs as villagers may not happen just due to design limitations.
Possibly could be said for snakes as well.
Bugs are bugs in the game and act as bugs, although we have Octopi as villagers, Octopus are actually intelligent enough to be on our level of sentience and therefore be able to be villagers. Even being able to be out of water for some time as well. Yes frogs and turtles are also villager/npc species and catchable critters (I think its mostly to do for cultural wildlife connotations and create a proper pond/river like ecosystem that feels more natural), but when you say you want sharks and insect villagers, those being fish and bugs, main staples for displaying and collecting. I think it breaks the concept of intelligence and specie boundaries the game tries to convey.
For me when it comes down to it, I think the best species that could be added into the game and also stand out among all other specie designs. Would be bats, armadillos and a tie between weseals/coatis/genets/civets . I have others in mind but they fall into the area of me being ok if they were npcs or not.