Above, I covered using the technique of viewing height map and a blueprint terrain together in a program such as Photoshop.  We worked on the blueprint over an existing height map, but note that the process can be flipped.  Roads and lot placement can be designed in one layer while the height map is developed from scratch in another. Each layer can be exported separately for use in a new world.

Getting the details of a heightmap perfect in a graphics editor can be tricky, but if you plan enough "give" for smoothing and adjustments, you can get a good approximation that can be refined in-game. It's much easier to plan out complicated worlds this way!