city terrains mainly are dependent on the amount of quality imports so your first step should be to layout a terrain plan in the sense of shape, and then either go for a full import set for doodads or go for a half set but make sure to accurately use small doodads to help shape the imports as they will be of different consistency from the standard models and will seem out of place otherwise. Tiles are often an overlooked element of a good city; imported tile textures can really help bring out the city aspect.
Don't use flat terrain even thought it is a city, and don't use blizzcliffs (really ever) but especially not in a city. Terrain mainly just takes time for it to be developed, just keep fiddling with it and using resources (with credits) until the image is what you want it to be in; persistence is key.
If you cannot find a model for something you want in your terrain a good idea is to look at some of the terrains in the terrain board and see either the model you want in someone else's display and can ask for it, or you can see how models you have access to can be stacked or combined to make a desired effect.