Yup. Sim City 4 Terraformer can take any grayscale image and make it into a region. it has advanced terrain smoothing options to make them smooth as not all images import into maps as smooth terrain. it's tricky to get the terraformer to work on Windows 7, they said it was possible since 2010, but I couldn't until August 2012, those were my first maps.
Sim City 4 is good. there is lots of sites for custom content. you can have one of your cities neighbor another one of your cities (in a region) there is tools that can create custom building and custom maps (regions) I have two Reshiram shaped maps, one is flat and the other, the altitude is based on what Reshiram looks like. here is an example of a map pack I created http://community.simtropolis.com/fil...mon-maps-pack/