Yeah, all the vehicles, structures, etc are in OP2_ART and can be easily changed once an editor becomes available. They could be local to a single map (exe memory hacks), or become a game mod (edit OP2_ART files on disk)
The tilesets we could change now. The map file itself specifies to the game what tilesets are to be loaded, so if you change that, then of course fix up the map so it actually looks nice (the cliffs are where they should be, etc) you could do it. For *any* map that is.