Stored in White's archives is a map of Europe. Wondering what the point of it was, I soon discovered this page here, which seems to be a definitive map highlighting the most major medieval European cities. (I'm guessing in excess of 50,000) Noticeably, the map cuts out places like Scandinavia and most of the British Isles, and areas where there were no major cities until we get into Byzantine territory.That is, it seems a large part of what's covered is land once occupied by the Roman Empire, and over which the Catholic Church had a stronghold. (With the fluctuating example of Iberia) It's a pretty good marking line, but what intrigues me is White has a map right here, which is all the largest (50k) cities in the former United States. More interestingly, Roman-touched Europe is comparable in size to to Non-Denom America. with the same number of these major cities.
The point is, West Europe and East America are almost perfect analogs for one another. Similar climate, population sizes, and the general feudal remains of an expnansive empire, only united by the big religious entity . A few differences pop up here and there, though. Most cities crowd around Italy and Iberia, while America is a little more spread out. Medieival Europe, as a whole is a very peninsular continent, who's steepest mountain ranges cut through the middle, so we see cities hugging the coast, Toledo and Paris being the most inland cities. America's geography lends itself to different phenomena. On the Atlantic, there's really nowhere to go laterally, and while the Gulf of Mexico does make a good transport, the shores are swampy and less ideal for ports than the Mediterranean. The main water routes are the Missisipi and Ohio rivers. The Great Lakes also act as a de facto sea, , but the region is also prone to harsh winters. Ultimately, the different regions of America have their own pros and cons, no one spot dominates.
As for other forest zones comparable to medieval Europe and America? Well, White has mentioned India as a region where feudal states tend to pop up. It's also a very easy-to-distinguish section due to its peninsular shape, and you can't throw a rock in the ancient world without hitting one of its major cities. You could say it's the third of these feudal areas. East Asia is hard to distinguishdue to China's incredible size and population--it's hard to know what to separate. If I had to establish a core point, it would be along the Yellow River, where historically most Chinese cities sprouted on--with the area around the Korean Peninsula being the upper limit. Like England, Japan would interact with this core, but only have a city inside it. The thin and rugged terrain of island nations limit their power in medieval times.
South America, particularly Brazil, would serve as the fourth great feudal culture. It's both easy and hard to imagine. It's structured a lot like the US; a heavily populated nation with a long racial gradient and a huge backyard; Its the jus the US has a west coast and Brazil doesn't. As I've mentioned before, the tropical climate brings India to mind. It can be somewhat simple to divide. The Southeast and Northeast are the most populated, so one could cut up a region there. On the other hand, the more temeperate and fertile south may yield higher populations, and absorb parts of Argentina and Paraguay. Meanwhile, Northern Bahia could serve as a buffer, where Northeastern Brazil starts to observe more Caribbean customs. The Caribbean is its own cultural zone really, but keep in mind we're working with areas of a certain amount of square miles and potential for twenty cities. (Western America, Central America, North Africa and the Northern Middle East are all examples of very populated and important cities that house impressive populations, but the arid conditions prevent too many from popping up.)
The last sizable forest zone in in Eastern/Europe, central Asia. The interesting thing is, this might be the one Old World location that wouldn't be a rerun of past eras. It's always been in flux, with conquerors as diverse as Byzantines, Ottomans and Mongols. And then there was Russia, which became a major power and a more solidified culture well after the middle ages, where in the 20th Century it was a major power. Would the steppes be conquered by nomads? Would someone make use of the black sea and the silk road? What would a post-communist medieval world look like? One could have fun there.