It shouldn't be that hard to implement them.. If you got models you could just simulate sitting in a car etc using car-like hudguns. Although cars wouldn't make sense, as you gotta jump in Sauerbraten. Planes/UFOs are also not that hard, as long as you got the models the coding part is rather easy

Anyways, I don't think stuff like that will ever be officially implemented in Sauerbraten, as it's a showoff of the engine and not a game that's gonna get more and more features..

Quality

For SP maybe, but how the heck do you wanna do it in multiplayer?

You'd need a servermod & every person needed the right client. But actually Sauerbraten is not the right game for such stuff

Indeed as you'll have trouble with the boundingbox also.

Boundingboxes wouldn't be that much of a problem. The only big problem is bringing this to the ppl.

The second big problem would be: who handles the vehicle entities? How are they added? Are they more like bots? Or more like items? Does the server have to handle them, kinda ruining the idea behind sauer's server/client system? Or do you hand it over to the client, possibly giving chances for entirely new cheats, such as wreaking havoc in insta with a tank?

They can be added as a barrel-entity in SP.

If somebodys gonna do this he/she'll probably male a new gamemode to prevent cheating. Things could fly around near the ground using waypoints, like bots, or just be static so when you get into/onto it your playermodel's gonna be changed to someone sitting in it and you could steer around using wasd or whatever...