I personally love evolutions and I'd like to see more of them but I also get why there's not a lot of it around; depending on the design of the game the workload might be the same as adding a new pet, and the paintbrush system inspired by neopets where the pet is just redrawn in a different pose/theme can be seen as a kind of evolution system.
Might get a little too close for Nintendo's taste of animals transforming into something else, but I think it should be included, give people something to strive for.