Bikes - Just simple implementation and few models (see car to do topic for links).
Sounds - I've found some car engine sounds, worth at least trying, new surface sounds, ambient (rain,wind etc) and switch to OpenAL soft to have reverb.
New Cars - See car to do topic.
New Tracks - I didn't do any in months.
Better road textures - Improve detail on some sceneries, use more textures, maybe add some blending between or some noise/trails.
Pace notes - 3D signs above road with corner difficulty info, bumps, jumps, stop/slow warnings, etc.
New gameplay modes - Career (3 paths with some predefined car/track setups on each stage) and/or collection mode (collect stars (or sth) in various places on whole track, probably also for best times).
Replays - Reduce the bloody replays size for new ones and add a button to compress older.