Not an editor as such. I'd rather like to see the feature of swappable car parts between each classes (GTR, TBO...) like suspension, rear spoilers and engines. No i don't mean that you could put a FO8 engine in the UF1, this swaps should stay within the same class. Or maybe even tweaking some parts, so you will go on a server that will limit your LX6 to 170bhp or maybe make your FZR wingless. Could be usefull for the leagues.
Another thing is modifying current tracks we have in LFS. You could make your own layout by opening or closing some corners, thus creating a whole new layout. Like Aston GP that turns right after T1 (on the AS National part) instead going left on the eau rouge chickane. And so on...