I just stumbled upon a mod that increases movement by 10% in friendly regions, and it seems to be a feature that should be implemented in an official patch. After expanding holdings by a fer regions it feels a bit too much like a chore to move armies around, and playing as Empire the raids from the north turn from a challenge to an irritant.
Maybe after controlling an entire province there could be another slot opened to upgrade roads, like +3%, +6%, +9%?
Chaos and Vampires could benefit from corruption (e.g corruption/20 = movement boost %).
I think roads were in every other TW (I barely remember Rome 2 so I might be wrong...), and it feels like something that a good emperor or dwarf king should look into