Some positions should be more defensible than others, or special type of terrain should not simply block movement or allow one way. This would be true for walls, too.
So, if we wanted to similate Hadrian's crossing of the Alps, then perhaps if someone attacks over the mountains in a map, it can be configured that the attacker only has up to two dice, or that the defender gains a third, depending on the situation or mapmaker's choice. So Hannibal can still cross the Alps, but boom! He takes heavy casualties in the process.
This would no doubt require some extensive modification of the engine, I think, but I thought it would be an interesting idea. (Did we have to modify the engine for embassies?)
The other thing is infrastructure like railroads or highways. This wouldn't require any engine modification at all, but basically regions along a railroad or a highway would be able to share borders with more regions along that highway it isn't directly connected to, except for critical points...also, maybe the region would only be able to attack other regions only the equivalent of two regions away, so access doesn't have to extend down the entire line.
Attacks along a major highway, or if bypassing nations, then there could be combat modifiers for that too. For example, a travelling army bypassing another region would be easier to ambush.
(I think this occurred regularly during the Chinese Warring States Era - states would try to attack another state not a firm part of their borders and would try to bypass another state. Ie. Qin tried to attack Hua, bypassing Jin but got ambushed by Jin on the way back.)