The problem in Gran Turismo 5 wasn't NASCAR. The problem was that it could have been better implemented and utilized. Would I want NASCAR in GT6? I am more "I wouldn't mind."
What I think would have worked better is if NASCAR was included as downloadable content (DLC) with a more complete package. All that would have been needed for NASCAR in Gran Turismo is for shorter ovals. Otherwise, NASCAR in GT5 had at least some sort of high-speed ovals on the superspeedway front. Motegi's oval was more like the good intermediate oval. I think you NASCAR fans can agree- some short tracks and some mile-long tracks would have sweetened the NASCAR deal. The game could have featured certain short tracks NASCAR races on like Martinsville, Bristol, Richmond, Iowa, and tracks like that. Mile-long tracks could have included tracks like Phoenix, Dover, or the boring New Hampshire oval. I also wished Gran Turismo would have made an effort to showcase NASCAR road racing. Say what you want about NASCAR- they do know great road courses. I am more likely to watch NASCAR road racing than almost any other event excluding the major oval races in NASCAR.
Besides these implementation issues, I also wouldn't mind more historic NASCAR cars. Give me some of the cars from NASCAR's Aero Wars (like the Superbird). I don't mind racing Richard Petty's #43 Dodge Charger. Maybe I feel like Intimidating others with that hot black and silver #3 Chevrolet. Maybe drive a few vintage Ford Fairlanes? I also wouldn't mind seeing Nationwide cars as well as the NASCAR Trucks.
So do I want NASCAR in Gran Turismo 6? I surely don't mind it. I just wish it were implemented better speaking as a motorsports enthusiast. All I ask is more enhanced material to spice up the NASCAR offerings.