What we've ended up thinking, based on what's seen as sufficient interest for 2 of the cars, and a fairly nice interest for the others:
Something similar to the Road to F1 series that DSR did a while back, 5 rounds, starting with MRT and ending with BF1, but no real restrictions in setup, and two (or maybe 3) races per round.
Full, yet still provisional details to be sorted out still.