Dont get me wrong i would rather a scooter game any day. But BMX has been around for years and they have pretty much no good games for them. It would be highly unlikely that scooters will be produced before a BMX game. But if it did, make it the same as EA skate 3 just with scooters, no need to be fussy i think. Also you said your friend is going to make the game........ It will take him years to make the game. Think of the end of every game when it has the credits come out, there are literally hundreds of workers on that one game, there is no way unless you can get help from some big companies. But give it a try if he wants to man, nothing should stop him. Btw I dont know if this is legit or not but there might be a new BMX game coming out, check the countdown.
http://www.bmxthegame.com