Traps. This would primarily be for multiplayer assuming there will be one. Traps would be like oil slick, changing the arrow of direction so they go the wrong way, tire lock, etc. I guess this could go under "power-ups" as well.

Being able to dismount and go flying off the bike would be fun.

A derby or something where you have to knock everyone off, last person standing wins.

Also, tournaments.

Wepon desingning