I like the idea of the Revo-Slot spec series. After the Goodwood series, which I really enjoyed, already having all the cars I built out for it, I have not had time to assemble and test unfamiliar cars for other series. As has been said, maybe only tires and gears to change. Everyone should be on equal footing.
For other series, I like Can Am, a spec Ford GT Slot.it series, and Trans AM.