I'm going to make some less-than-expertly analysis on this.
I predict the total number of cars in GT4 may be anywhere in the 500-750 range. Maybe 800. The parity among cars will be maybe 30% Japanese, 30% American, 25% European, and the other 15% will be every place else. I'm not going to use "bias" to describe the majority of cars in the game. I put the Japanese and American car counts at just about even if you consider that there may likely be as many American cars as there are Japanese. How PD hopes to work the percentages is up to them. I think we may see many more American cars ranging from low-powered models to muscle cars to true sports cars to race cars. They may provide the Sean H.'s Chevrolet Cobalt Time Attack car from last year (I'm thinking about starting a thread on this subject, so stay tuned). We'll see a variety of cars. Maybe GT5 will have about as Enthusia in terms of diversity among different cars. We may see a good number of cars from past, present, and maybe future (concept cars, that is). The diversity among Normal, Racing, Concept, and Tuner among the different cars will of course be pretty high for normal cars. Expect maybe 55% Normal Cars, a strong 30% of race cars, a 10% contingent of Tuner-type cars the world over including Drift cars, and the rest as Concepts.
It may be just the amount of diversity we'd expect from Gran Turismo. I don't think we're going to be looking at a deja vu of GT3 in terms of an old game on a new system because in terms of cars, GT3 was a disappointment over GT2 on the car front. But did many of you notice that a number of the different car companies from GT2 still carried over into GT3 with a few new additions? My X-Factors are on what cars COULD be in GT5, how much detail will we get to see from all of them, what kinds of touches will be made to make them more realistic (for example, functioning windshield wipers, animated drivers in the car like in Forza Motorsport, the dreaded "reverse lights" everyone wants, etc.), and how many can be on the track or be utilized in a number of disciplines. We may have just enough of each car company from the represented country in which the worldwide perspective of all the automobiles won't be an eye sore for the discriminating GT gamer. If they could do 1,000 cars and have as many of them without too many complaints from the public at large, I think we'll all be happy.
Well there. I've gone through this post without saying "Japanese bias." PD may surprise us again. Never know. But never, never, never, NEVER EVER just be silent about the kinds of cars you want to see in GT5 and of the parity among the different countries. Again, it doesn't mean PD will readily pick up your commentary. It just means that any Internet suggestions you hope PD can pick up and use in developing a game, it could happen. Just ask the Buick Grand National, Fiat Panda, inclusion of Nürburgring Nordschleife, inclusion of Le Mans... I'm telling you. NEVER give up! Make your voice heard. Maybe PD will take your idea and probably think about this post I made on the cars we could see in GT5. You have my word.