I'd like to see more vintage and classic cars, more pre-1960's racing cars. Having built some of their Formula I kits, and enjoyed them immensely, I'd love to see Tamiya apply their talents to something like an Auto-Union, or Mercedes W163.
As far as vintage cars are concerned, Rolls-Royce's Silver Ghost was the the model that really established the reputation of the company, and were used by the British as armoured cars in WWI, so there's potential appeal for the military modellers as well. However, if I had to nail it down to one model I'd like to see Tamiya, or for that matter, anyone, release, it would have to be this:
[img=left] http://www.n-connect.net/benetton/cars/car-2000-06.JPG [/img=left]
A 1935 Auburn Speedster. At least two companies that I know of have felt there was enough demand to offer full sized replicas, and you've probably seen them in just about every film set in the 1930's, yet the only plastic kit I've seen anyone offer is the wretched Pyro/Lindberg offering.