It's not my original idea, but I haven't seen it mentioned much yet. This is in fact one of those features in a rather infamous game called Imperial Glory that I wish was included in ETW...
Right now if you are playing as e.g. Britain and you capture Hannover, the troops you recruit there will wear the British uniform. However, it should be possible to create "recruitment zones", so that a region will give you access to particular units. In other words: troops you recruit in Hannover will be "Hannoverian line infantry", with their typical uniform (and maybe even flag?).
I'm sure there is a historical basis for this. And besides, I think it would look much nicer, adding more colour to your armies
Maybe it is also possible to add a script to the campaign so that you can recruit a (limited) number of regional units when you are the protector of a nation?
I'm not sure how to do it and if it means a lot of work. The units and skins are already in the game, even if they're not always accurate. It's probably also more realistic to confine this to the minor factions.