Interesting idea, if I'm understanding right it'd be something similar to the rally races in the Special Events section.. staggered start with each person leaving the grid one by one maybe 5-10 seconds apart, but given we'd all start the timer at once you'd take X seconds off the final times depending on when each person leaves?
Car wise how about any rally / low powered race car that can make 1200kg / 325hp that can take dirt / snow tires but running on Racing: Hards for tarmac? That would sort of cover 2 of the possibilities you mentioned in one
, for street cars it might be worth specifying a type of car or maybe an age range or something (like pre-1975 japanese cars or 4WD only or something).
I really don't mind what cars are allowed though, I'll try anything at least once
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Addendum - I just remembered the two older rally cars, the '74 Mitsubishi Lancer and '69 Nissan Bluebird rally cars.. they're awesome little cars if you don't mind the lower power!