OK, then. I had a quick look at the UUs (at least the european ones) and I had some thoughts I would like to share. Just do with it what you want.
Italy: I've read that the Condottieri (the term actually refers to the commanders, not the soldiers themselves, but I don't think that really matters) held on to outdated tactics and traditions and proved themselves ineffectual against the more modern armies from other countries. Perhaps it would be an idea to make it an earlier, but worse, cuirassier instead of knight. So Italy will initially have an advantage over the other civs' knight, but after those modernize and get to cuirassiers, Italy will be stuck with a less effective cavalry force.
HRE: Since the first UU is the Landsknecht, with the Doppelsoldner would end up with two medieval melee UUs. That's a bit too close together for my taste - especially since technically a Doppelsöldner would just be a special type of Landsknecht. However, in the form of austrian Habsburg, the civ represents a great power for quite some time longer. My idea: Hungarian Hussar instead of cavalry. Since Hussars are light cavalry (that is, everywhere except in Poland), the visibility or movement cost promotions would lend themselves.
Prussia: The combination of Panzer and Howitzer gives this civ too much of a 20th century focus, in my opinion. After all, it starts with Frederick II as its first leader. When looking at the seven years war, especially the english speaking world focusses on the fighting in english and french colonies, but for Prussia it marks the maturation into a power to reckon with: Severely outnumbered it prevailed against russian, austrian and french attacks and the discipline of the prussian infantry became an ideal for a generation of european officers. Thus my suggestion: replace either rifleman or grenadier for prussian UU.
I know too little about most of the other civs too really contribute anything useful, I fear.

But there are some units on the list that I really like.