Regions don't have to be taken simply as geographic restirctions. Most of the worlds auto manufacturers are controlled by massive parent auto groups, so why not make divisions among them. GM, VW AG, Ford, Diamler-Chrysler, Toyota, Honda, Nissan and so on.. GM, Ford and Chrysler make up the majority of American manufacturers, VW seems to have Central Europe locked up and everyone else has thier niche'. So say that Ford, GM and VW AG each comprise 15% of the lineup, Probably slightly more for VW if PD can land Bently, Bugatti and Lamborghini. Then Diamler-Chrysler and Japan Make up 35% - 45%, and BMW and the independant, semi-independant ( IE: Porshe, Lotus) and tuners make up the rest. The percentages may be off a little but the idea is simple and since most of the parent companies have brands in most world regions, It could work!
I left Fiat AG out because aside from Ferrari and Alfa Romeo, they dont have many interesting brands. Besides, as much as we all want it, there will be no Ferrari's in GT5, or any GT. There will likely never be Porsche's either even with Ferdinand Peich running VW.