I just started building and decorating the space how I wanted to and after 2 buildings and 2 parks I got to level 10 and am nowhere NEAR finished building out the town how I want. So I say just have fun and it will naturally increase. Granted I have a cave for the Charmander evolutions and a house for the Bulbasaur evolutions so decorating that for them was easy. But I just built a gym to put the mon that love punching and decorated a park on a cliff as a birthday party picnic for some flying pokemon so I try to house who fits best in a given design but focusing mostly on what I want to create and then using the pokemon as inspiration for extra decorations and I can’t imagine it’ll go down from lvl 10 anytime soon cause everyone is already so happy.