Just wondering if there could be categories for shorter runs. Maybe something like unlock rank 10 and 5 or 6.

Tbh I think if more burnouts games get more categories it helps bring in newer runners also are easier to run faster because alot of the burnouts take around 2+ hours to beat and that can be a pain of a run to do. So yes they should be added whatever ranks you think for miscellaneous category would be best fit or even every rank if theres not to many.

Should bonus cars (for Burnout 3, Madden, etc.) saved files be allowed for runs or just use the base cars?

If its not any DLC/cheats/glitched cars and its unlocked or in the game its self u should be able to use it. Also its just bonus cars u unlock so i think ur good