I also have still a few sims I have to check out further. Some Rally stuff and Truck racing, and NASCAR. I heard Truck racing is quite big in Australia, too, right? I mean the real HGV trucks. But there is also a series called "Stadium Super Trucks" which is rather entertaining. Thats custom build buggies, similar to what they use in the Baha racing in California - only the Stadium trucks race on proper roads with jump ramps on them. I would like to have a sim model of that, they must be really crazy to drive with that 50cm supension travel and all that.
Another interesting idea for a racing game would be the figure 8 racing the crazy Yankees do, have you seen that yet? If not, take a look on YT. :)