I've been working on a http://www.gamesas.com/index.php?/topic/1134434-wipz-hoverboard/, but a bicycle would also be really cool. There are two problems with vehicles: appearance and collision detection.
Appearance:
Since there's no cycling animation and no working bicycle model we can pretty much forget about thirdperson perspective until someone goes ahead and makes them. Tilting the bicycle's angle according to the terrain is another problem. It can be done, but it's extremely hard to get to look right. Tilting the player is impossible, so it'd look pretty weird anyway. Forcing first person perspective is your best bet for making a bicycle mod that doesn't look very strange.
Hit Detection:
Moving a vehicle means either setting the player speed higher and then moving the vehicle to the player, which will make you hear footsteps when using WASD, or disabling the player controls and using your own setup, which also disables collision detection. Using your own collision detection is an option, but this might not always work, and will probably be a bit wonky. So you won't get a smooth ride anyways.
The reason why I've made the hoverboard is that it avoids some of these issues. It doesn't need to tilt, and it can soar above low objects to avoid having to hittest against everything. Incidentally, the player already has an animation that's perfect for the board, which is the current melee stance. Until NVSE comes out, making any other type of vehicle will be very very hard.