Was there ever a discussion of a city terrain variant rule? It seems to me that most large cities (Endier, Imperial City, Ilien) are plains surrounded by plains and as such not suitable for connection by trade routes. Would it not make more sense to make a separate type of terrain, city, so that cities could be connected with surrounding provinces?
Well most of them are on the coast or by rivers for probably exactly that reason, although roads in plains are dirt cheap which allows trade fairly easily.
Of course if you split the city from the surrounding province (the imperial city) - then it should be a different terrain type, in this case city.
Personally I dislike splitting the city out - from an income point of view if you split your L4 province into 2 L3 provinces you will increase your take which is both blatant munchkinism and simply wrong economically (it should raise less cash due to the increased inefficiency of two sets of bureaucracy)
You could add different resources to provinces if you liked - the mechanic is normally assumed to be covered by terrain but you could add a more detailed system if you wanted. So instead of being able to trade between hills and plains you can trade hills (iron) and plains (wheat).