I think the next step after creating a ton of cars and vehicles the next step would be planes. I don't know much about programming, but if they create a way to simulate lift it shouldn't be that hard right? But the next problem I see, is with the naming of it all. After all its called Drive not Fly, so would they create a second game/demo ? (if they do decide to add planes, etc)
Yeah, new game would be cool, it would separate it all, I can imagine the trailer: From the creators of BeamNG. Drive and Rigs of Rods comes a NEW game..: BeamNG. Fly!
It would certainly take some time, but I think planes would be really fun. BeamNG's physics are already spectacular with cars. I can't even imagine how awesome the physics would be for planes.
Gliders are technically possible right now if they are spawned in the air. Planes with engines aren't possible right now, but will hopefully be possible in the future.