The terrain might need to be represented by a mesh instead of a heightmap. That way, you can add detail where it's needed. I'd need to make an editor though cuz I only have MilkShape 3D.
[ed] You could then make mountains and ridges. But you'd need some fast way to determine heights/collisions for each unit without having to check for collisions with each map face.
If deformable, you could even dig bunkers and stuff.