1) Lock the research behind later vanilla technologies, instead of being available from the start
2) Maybe originally have a lower speed, but through more expensive researches the speed could be increased?
3) An option to play Tony Hawk music :)

2) I like nr 2. But would there be a way maybe to set the speed per player? Like personal experience, the more you practice, the more you can adjust. (Not only increase, a way to ride slower when experienced is good too)
1) As for research I would say after rails, after a few of braking force.
4) And I would loooove to see some sprite. I doubt in my abilites to help on this one.