Also here are more ideas:
When you buy the pet, you can buy it in different colors.
Make some pets limited time offers, such as a reindeers in the winter, and dragons in the summer, etc.
New house types dedicated to pets (such as a dog themed house if you have a pet dog, a fish themed house if you have a pet fish, a cat themed house if you have a cat, etc.)
After awhile, maybe 3-4 months, the pet has babies. Then after 5-7 months, the pet dies. The babies look exactly like the parent, and the amount of babies it has depends on what level the parent is, and how good you treated it.