Few possibilites as alternates would require more tracks as well would be of course Indy car, Nascar or Supercars but when they demonstrated modding on their video they showed space ships implying that you could add additional car types something which has not happened to date.
It seems that might be a future option which would allow pretty much any possibility but I think more tracks would add greater option for diversity in the future the game has multiple options to expand on a great management sim.
One which might be very interesting for those in to career mode type scenarios would be karting since many driver of all types of race cars cut their teeth on the beasts. You could start as a junior and work your way up budgets would obviously be low but so would costs etc.