Shrimp is a good one that’s already sort of popular. Freshwater fish are already common so why not?
There’s lots of herbivorous beetles that could have potential. Darkling beetles are pretty common and easy to breed.
For another insect contender there’s cockroaches, but even for the more photogenic species, getting over their reputation of their pest cousins can be difficult and their populations can easily get out of control, people would have to be willing to cull them regularly.
Ladybugs could be appealing to a lot of people, but their diet may be an issue, I’m not sure if they’d accept any alternatives to the live prey they’re used to. Also they can fly, which adds complications.
Phasmids don’t require much aside from water and a plant with edible leaves to hang out in, but I’m not sure if a pet that’s hard to see even in plain sight is something people can get behind
For another aquatic example, echinoderms are somewhat overlooked by saltwater hobbyists compared to marine fish, coral and crustaceans. Starfish, sea cucumbers, sand dollars and sea urchins are pretty cool on their own.
For vertebrates, maybe shrews, toucans, and hummingbirds? The little penguins in the Eudyptula genus might also be possible, I’m not really sure what sort of needs they would need met.