Yes I agree with you that class differences would also be felt in the economy of each region, hence its good because then each province is divided between being an economically important area or an area that provides better troops, as well as other factors.

I would suggest that we assume that whatever we come up with applies to a human world, because it is impossible to make something for all the potential permutations that may come out of other people worlds. Hence the importance is coming up with a system and then people can adapt them for their game world as required. I may do a test map of one province so we can start working out the variables within a specific map and hence can use that as a guide for our theories.

I like the thought of working out population growth out of variables like you mentioned in religion, health etc. Problematic ones are emmigration/immigration (because it requires a marriages of information from several kingdoms. Food production we can do by working out the percentage of land that is arable and hence work out a figure. However it becomes more complex than just regional food production figures, as food often is grown in one province and is sold in others through domestic trade, thus arable land must be taken on a national level and permeated through each specific region. Hence population growth then becomes whether nationally there is enough food.

1 sq mile of land in medieval times has been estimated to be able to support 180 people. Hence if a country had a population density of 180 people per mile then every single inch of it would be farm land, which is impossible.

Therefore because food would likely be treated nationally because of trade, we can assume again that there is no regional limit on population. However that is only in theory as I assume there would always be a limit of the flow of trade into specific regions, also food prices would be extremely high for those living in areas that are food poor. Anyway I'm liking the way your thinking its getting me thinking more. I would suggest we leave magic out of it, and when we come up with something that works, we can state that through magic, land could perhaps 1 sq mile of land could support more than 180 people per sq. mile.