I think if you can't have Le Mans, give Sebring some thought. I played it in Sports Car GT for the PC (the better version), and the thrill of high speed is incredible on the front straight and that long back stretch before heading into the final few turns.
I would like to check out Suzuka and better improve my driving around the Casio Triangle, you know those two SLOW corners before the front stretch? If GT4 was putting that into the game (highly likely), I would like to see an easier route into the pits. Because the Suzuka in Le Mans 24 Hours was tough, and I usually missed the pits.
With the NFL in town here in Houston this weekend, I seen commercials for the Pro Bowl in Hawaii. And that reminded me. Why not have a rally race in Hawaii, and then a street course in Honolulu? It would be a new-age Tahiti. I still think Houston, Texas would make a nice venue for a GT race. If not for GT4, GT5 will have to be an interesting choice. Houston- America's most underrated world-class city.
Go Texans!