Bigger levels? Memory usage will go way up. Right now storing the trails for a level takes up about 30mb of your computer's RAM and good chunk of CPU cycles. That goes up proportionally with the total area of the level.
Multiplayer with more than 8 people? The trails will look different on each player's screen. The limitation right now is that that server tells every player connected about the position of every other player in the game. Making the server only tell each player about the position of other nearby players will mean less data flying around and more people can play once, but not all the trails will be drawn. Every player will see different trails in the snow depending on who is riding nearby.
iPhone/Android version? The trails need to be stored a completely different way. They can appear behind the player for a few seconds, but then get erased as the rider moves on and leaves more trail.