Char you need to add a normal bike power-up so that the race aspect of the game can still be maintained when switching between helicopter and bike and visa versa. e.g. ride some track, hit the heli and reach a new ridable only section of track and have to wait for the timer to finish (somebody else has caught up to your amazing helicopter run and over takes you on the bike section)
A motor bike would be pretty cool. It would need to be the same speed and height as either the MTB or the BMX other wise some tracks may be unplayable or the motorbike would be to overpowering. I think a trike would be cool to have too!