I think it could be cool if we could stop the engine temporarily in midair and hover. It would let us make sharper turns when for example you're going up and then need to make a 90 degrees turn. I'm not sure if the physics make sense but I think the feeling would be great!

The restarting of the engine would have to be perfectly timed with the release of the key though, unlike the boost.

Oh yeah, and the "time left" label in the bonus rounds is truncated.