I wrote up some thoughts on how to design the PLC. I’ve been thinking on and off about this for the past couple years, so with the announcement, I figured it would be a good time to write things up.
I have not, however, opened the game since September 18th, 2021. So my knowledge of exactly how everything works right now is a bit iffy. I do follow the competitive scene, but there are many parts of the game that just don’t come up in tournaments.
I usually don’t have enought patience to read posts that long but I made an exception this time and I don’t regret it 
Nice ideas, I agree with most of them.
Interesting ideas, here are some thoughts:
“Especially during the partition period, Poland has a long history of rebellion and revolution. This is an element that should be incorporated in the game in some way, as there are many interesting mechanics that could be spawned from this. A basic and sufficient implementation that I can think of would be something like the Poseidon mechanic of AoM. When a building is destroyed it spawns some Kosynier units - maybe this also occurs when villagers are killed. Alternatively, there can be a mechanic where villagers can be converted into Kosynier (or other) units quickly, then turned back, without a full Revolution being used.”
The US-Texas federal card “The Alamo” works in a similar way to the Poseidon’s Militias, it could be implemented as a default bonus for Poles, although somewhat nerfed compared to the Texas card. Also, the Polish Settlers can share their build limit with the Kosynierzy, so that balance is maintained when converting Kosynierzy into Settlers with a card or other mechanic.
“Continuing on the theme, it is possible that the PLC will be a revolt focused civ. Most of my play was before the revolt mechanic got expanded to its current status, so I don’t really feel like I am in a position to provide interesting thoughts on how this could be done - seeing as I don’t even really know what is in the game now when it comes to this (tournament games rarely see revolts, and watching those is my primary consumption of the game these days).”
I find this mechanic of revolts interesting, it may be that the Poles have a similar feature to the Mexicans. For example, to ages III, IV and V you can choose to age up between 5 politicians or a revolt, which transforms the Settlers into Scythemen (Kosynierzy), along with send more cavalry and/or something else the older the age.
“Age of Empires 2 has a Poland civ, which has a unique mill building called the Folwark to represent the food production. This is also a possible addition to the game, others have proposed something similar to the Mexican Hacienda”
I think Folwarks can work very similarly to Haciendas, but can receive different bonuses through cards.
Also, the Polish Settlers can share their build limit with the Kosynierzy, so that balance is maintained when converting Kosynierzy into Settlers with a card or other mechanic.
I think the Kosynier would not be an otherwise trainable unit, with the conversion implementation, and would be a form of settler - so the built limit should definitely be shared.
I find this mechanic of revolts interesting, it may be that the Poles have a similar feature to the Mexicans.
I think it would make more sense for the revolt focus to be late game, to be representing the partition period. However, there were still many Zaporzhian and other such revolts throughout the entirety of the PLCs existence, so it wouldn’t be completely off to have a full game revolt focus.
Maybe from II Age you can train Kosynierzy but only in the Town Center, and later in the game, in III or IV Age, through a card or something, you can train Kosynierzy in some other building. There is also the option for the Kosynierzy to be like the Sansculottes, Settlers who can gather resources and fight well, but as long as they are Kosynierzy their gather rates are reduced, so it is necessary to convert them into Settlers to improve their gather rates or something like that.