It's just an idea. Feel free to inform me as to why it is implausible.

Every bike type has a separate leader board, as well as an overall leader board with an icon detailing which bike was used. This way, when new bikes are released in DLC's, there wouldn't be any negative repercussions to making these bikes available for earlier tracks. This would encourage replayability on all tracks with alternate bikes.
The only down-side I can foresee is the amount of storage it would take to make the top 10000 replays available for every bike on every track... but would that really be an issue?
Lemme know what you think!