I'd like to see a GT: Professional spin off. Rather than doing races and winning money, you would start in a low series, and get hired by a team. You would drive for the team, and by getting better and better results, you'd eventually get hired by another team and move up to the next class. You could choose what teams to take offers from, and, also, what kind of racing to do. For example, you could start as an independent Autocross driver, and move up the SCCA ranks, first being a club racer, than onto a full team, and so on. Or, you could go Rotary Rockets, then up to Formula Atlantic, and into CART. You could even work your way up in the NHRA world. Other series such as DTM, JGTC, F1, and so on, and their feeder series, would also be represented.
Eventually, you would become a team manager. You could hire drivers and crew for your team, negotiate sponsorship deals, buy parts and cars, tune the cars, and manage other team details. If you so desired, you could take over from the AI drivers and run the race yourself. In this series, too, you would start low and move your team up the ranks, and get better sponsorships, cars, and drivers.
Online play is a must, of course. Imagine starting as a driver in a low class, working under a team managed by another player, with experienced "mechanics" tuning the cars, and someday having drivers under your command as team leader.
Another idea I like is something like the "cribs" mode seen in some of the recent sports games. You would be able to build and upgrade your shop. In addition to the "Auto Modellista" style visual mods, however, the tools and equipment would actually be used in the game! As in real life, any mod or work would take time, and therefore, only a limited amount of work, including repairs, could take place between races. Improving your shop would reduce the time the work takes. It would also allow you do perform modifications that would otherwise not be possible. For example, you would have to aquire an engine lift before doing an engine swap. Also, it would apply in-race. For example, better racing jacks and air tools would decrease the time spent during a pitstop.
Of course, your team would generate profits. And these profits could be transfered into the independent racing as seen in the normal GT series, to finance your own cars.
It would be, overall, much like a sports simulation game, except in the auto racing world.
I bit foolish the speak of, but I'm gonna say it anyway: I'd like the see a GT spin-off in the arcade where you would race in an actual full-sized car cockpit and the surrounding windows would act as the monitors, giving you a full view of the course like in real life. They could even go ahead and model some of the car cockpits after the real cars in the game, and the race would feel like how it would in real life. And aside from the real cars, some fantasy ones would be pretty nice. Not crazy fantasy concept cars, but sorta more like stuff along the lines of the Affolter-tuned, Sposek-tuned Lamborghini Diablos' and stuff like that. That'd be sweet!!!!! 
. Gonna be a great holiday season.
Meow! (='.'=)