fort, that gives me an idea and I'm just typing it out as it spews forth from my head... Feel free to tweak as necessary.
How about this, you start with the same "countries" and starting units. Except each unit is captained (generalled?) by a single person. Each country also has a leader. For example, Austria would have four players, the leader plus three generals, one for each unit.
The leaders may all talk to each other like normal. The generals and leader of each individual country may talk to each other. Generals may not talk to other countries' leaders. Generals can talk to other countries' generals if they are adjacent.
Builds - The leader decides what will be built, and which of their generals gets that unit. So each general can gain more units to control.
Disbands - The leader decides which general loses a unit. If it is that generals only unit, the general no longer has any units, but could be brought back in later on if the country gets a build. But would not have to be.
Movement/retreats - the leader will coordinate what moves the generals should order, but it is up to the generals to decide what to really do. A general that doesn't behave may lose their unit on a disband, or not get awarded additional units on builds. Or they might.
Overthrow - if all the generals of a country unanimously vote for one of them to replace the leader, then it is done. The ascending general's units are then assigned by the new leader to the other generals. The old leader is considered a unitless general and could be brought back in.
Note, this means that if a country is down to 1 unit, that general can just take over as leader if they choose to do so.
Defection 1 - A unitless general can be given a new position as general of another country if that country's leader wishes. As you gain lots of centers you may want more than 3 generals.
Defection 2 - A general with units may defect to another country, but may only take half, rounded down, of their units. The remaining units will be reassigned by their country's leader. (This seems way too powerful because generals would just defect to flee a sinking ship. Or defect to help push another country, that they are now part of, to a win. Maybe defecting generals can only share in a draw and not a win so by defecting you are removing your chance at winning?)