Wouldn't mind more Gr.B's. The list is rather dated right now, plus adding more would give them a reason to bring back Tahiti Maze and Chamonix
I do also think what GT7 could really benefit from in terms of race cars are vintage models. In GT6 we at least had the Camaro and Challenger touring cars, that when tuned right, were competitive against each other. I would love if a Gr.5 was added to the game that featured solely these type of vintage race cars, and there are a lot of muscle cars that could be converted into Gr5 touring/race cars. The Camaro and Challenger are obvious ones, but I think the '69 Ford Mustang, '70 Dodge Super Bee and '70 Plymouth Superbird could also be good additions.