I made a really rough sketch of what I envision this planet looking like and added some coloring real fast. Here's what I came up with:
My original idea was similar to Glassman and Chewie's and this really is very close to Glassman's but with a few little tweaks. The most apparent one is the Imperial base no longer being connected to the city. My reasoning behind this is that a lot of US military bases tend to be on the far outskirts of a city or a few miles away, so they can have their privacy. Imperials would still patrol the city and hold its wall, but their main base of operations wouldn't be connected. We could add a small little outpost inside the city if people wanted to.
I went with a walled in city, as it will be easier to transition from city to wilderness if there is a solid dividing line, and it will make things easier on me and the diorama team later on. The easy in-world reasoning would be that it is a harsh wilderness and they are trying to keep everything out. I figure the city had expanded since the wall was built hence the settlements on the outside. Credit does go to Glassman on that part, I liked that about his drawings.
As for the river, I think it could be a really cool feature. It would be very hard to make it look realistic though, so we could just as easily make a dried up riverbed.
I like the idea of mountains, having different levels of terrain could look pretty cool. I added a hidden Rebel Base in the back corner there, as well as a small village near the river and the edge of forgot to draw.
At this point the wilderness will be whatever is decided upon.