For modern series definitely IMSA Weathertech Championship, would be cool if it included the Michelin Pilot Challenge as well. And, IndyCar.
For "classic" I'd pick mid 1990's CART Indy Car... say 1993 - 1997. '95 was 25 years ago so I think that officially qualifies it as "classic" now! Actually... I think Papyrus Indy Car Racing II was the 1995 season, would be kind of neat to have a modern remake of a classic sim as well! I still think that era of Indy Cars were the coolest they've ever been, and the schedule had a few more ovals than they do now (Homestead Miami before they reconfigured the turns, Milwaukee, Phoenix, New Hampshire, Nazareth, Michigan... anyone remember the "roval" in Brazil?).