Scawen and co probably reading this with a sense of dread at the sheer amount of scope creep that would happen if they even started to make a basic track editor.
However when I think about it, sure development time could be spent on Physics, but LFS already has great physics, time could be spent on cars, but LFS already has a large variety of vehicles now, a new map sure but if we had a map editor we'd probably have 100 new maps same as vehicles.
So, actually, yes I think a map editor would be the next natural evolution of LFS.
The execution however I think is also important, I should imagine map making is pretty involved and most of us are not game developers. Perhaps some procedural tools to complement standard 3D objects would be good such as the ability to draw road curves and have scenery etc follow the curves. There are many games with world editors that do these kind of things.