Progression: The further you sail, the better / newer items you start to unlock.
Sails: More sails equals faster sailing (err.. or drifting). Don't have to necessary move the raft. Just an internal distance counter.
Storms: Clouds building on the horizon then you hit a storm. Huge waves would be legit (although maybe too complicated to simulate) but just heavy rain, wind, and parts of the raft having a chance to break off would be great. Might mess with your distance as well.
Left Field: Ice-burgs.
Keep up the good work!