I am just thinking - do you know the game Insane (aka 1nsane)? The most prominent feature has been the "endless" maps. Which means, if you left the map to the left you instantly entered from the right. Same for top and bottom. Like a small globe (but without a bent surface). Is this possible to do in beamNG, too?
I don't know how much you can mod this game in that way. But if it is possible it is not trivial. But if anyone gets it working, serious kudos go to them.
I was thinking about something like (invisible) portals maybe, in case you can't make the map itself "endless".. but then I don't even know if the engine supports this. Where portals would definitely be a fun addition anyway.
