Yes you can totally sell new versions if they expand the engine/editor in ways that allow for new games. However you can think of it this way. The game shipped

Yes you can totally sell new versions if they expand the engine/editor in ways that allow for new games. However you can think of it this way. The game shipped is a point in space. The possible mods to the game are nearby points in this gamespace. Given tools dictate the work involved to get to that new point in gamespace. The better the tools the larger that isosurface is for some given max amount of work. The problem for game studios is that the things they want to sell might exist within the space that is work path accessible to the mod community.