Don't think there is an easy way to make each city use a different livery for it's troops without filling up the list with a lot of duplicates. The units can swap randomly from the selection on the texture ..but you would have to make each troop type a unique unit for each city to allow it's own livery.

So you could have City_a_hand_gunners, City_b_hand_gunners and so on. The combat stats would be broadly similar ( or not, if one city is famous for it's swordsmen or hand-gunners or whatever ).

The tricky bit would be the recruitment. We would have to limit recruitment of certain units to certain cities to maintain the uniformity and sense of it. Otherwise, you could recruit city a units in city b and every other city you capture. Spoils the overall effect to a degree. It's a nice thought ... but I don't know how well it would stand up to the acdtual reality of gameplay.

We could probably have some elite units recruitable only in certain key cities ... like the old RTW Spartan Hoplites. They could have a different livery from the 'ordinary' version available everywhere in the Empire.