Suggestion: We don't necessarily have to confine Tatsu to the world of the possible! Wild animal pets - like deer, foxes, wolves, lions, tigers, bears, komodo dragons, the sky's the limit - would be really fun! I'd love a few types of foxes and tigers, personally - Siberian tiger, Bengal tiger, fennec fox, red fox, grey fox, etc. I think that would be very popular.
If you'd really like to keep pets to the "possible" for one reason or another, I still think pet categories can be expanded - rodents! Rabbits (and there's a lot of different breeds of rabbit that look completely different), hares, chinchilla, guinea pigs (again, different breeds that look completely different), mice, rats, degu, gerbils, hamsters, etc.
Reason: I think it'd be fun to branch out a little bit from the cats/dogs/birds niche!
Images:
https://www.coopsandcages.com.au/blog/ultimate-list-rabbit-breeds/
List of full rabbit breeds
Degu!
Fennec Fox!
White Siberian Tiger
Bengal Tiger
Username: Sekah