Recently, the addition of the PLC to AOE3 was announced. With this being the last major European state (depending on how you count Germany - really Prussia and Austria should be seperate) during the games period to be added, I have given it some thought in the past. For context, I have not played the game since a bit after the release of the African civs. So I played the game with those, but have not touched the game since the Europe expansion with Italy and Malta. This is relevent, as I am aware that there are PLC units in the Native rosters on the maps added in that expansion, I do not think this will be majorly impactful, but I will comment on these where I know of them.
I think it is uncontroversial to expect the PLC civ to have a heavy cavalry focus, in both senses of heavy. However, there is a lot more that can be drawn on that could be included for interesting gameplay and mechanics. Moreso than any other civ in the game, the PLC was a multi-ethnic state. There is obviously both Polish and Lithuanian influence, but beyond that there was also a significant populatian of Cosssacks and Tatars, this being especially noticable in the military organization of the state. Many other cultures were also included, but I suspect that they are not significant enough to have more than a token presence in the game. the political entity of the Sejm is also likely to make an appearance in one way or another. This is the pseudo-democratic entity consisting of the male nobility of the state. During the end of the period of history the game covers, the PLC was dismantled and partitioned by Prussia, Russia, and Austria, which should have some representation in the late game.
I think in most RTS games, this would be the most in depth and vital discussion point - What units can the civ produce. However, there is so much more in AOE3 that I think this can largely be sidelined. My guess for the general unit composition would be a core of very powerful heavy cavalry backed up by cheap and weak light units of other types. The most obvious choice for the heavy cavalry unit is the Winged Hussar, which I think is likely. However, I am aware that one of the Euro native units is the Winged Hussar, and they may not want to change that. There is, for this case, an alterative option in the Pancerni. Pancerni were the vast majority of the heavy cavalry of the PLC, with the Winged Hussars only being the very elite (and wealthy, moreso) divison of them. Either way, one of these is guaranteed and the distinction is not particularly interesting gameplay-wise.
The much more interesting speculation for military is the other units involved. I suspect the other cavalry will almost certainly consist of a cavalry archer unit, most likely the Lipka Tartar (who I believe are also a native unit, so a name may be changed somewhere). I think there is space for a third cavalry unit, which I am hoping will be a hybrid ranged/melee light cavalry - probably called Lisowczyk. This would an individually weak unit, I think something like a hybrid of the Steppe Rider and Keshik of China. On the infantry front, I suspect there will be a musketeer unit flavoured in a Zaporzhian Cossack theme but mostly be a standard musket. I don't believe there were any skirmisher formations in the PLC military, so I suspect there will be none of those. Likely the normal pikemen will be used, but I also think it would be interesting to have a Kosynier unit. This would be a cheap and weak melee unit - something like the Russian Streltsy but melee. I think the Mexico civ has something like this, I have not actually played with them ever. I suspect the PLC artillery will be completely standard.
I think the age up mechanics are the most likely spot for us to see the Sejm implemented in game. Each age up will be represented by a vote in the Sejm, each giving different benefits. I think this is a simple, clean, and reasonably flavourful solution - although a bit of a boring cop out if not expanded on more. An easy addition to this is to have some sort of Veto mechanic - representing the right of Veto given to each noble voting in the Sejm - which gives a very different benefit. An interesting take on this would be to somehow have the Veto cancel the Age Up while granting some other significant benefit. So you pay the resources to Age, but the age up doesn't occur. This leads to either needing to re-pay for the same age up, or you are in a pseudo-age up (so a veto to age2 keeps you in age1, but you are able to go right to age3). I think re-paying for the same age up is cleaner, and likely a simpler thing to implement and balance. Although it may be hard to make it powerful enough to be actually viable. I think it is a very gameplay-flavourful way to represent the PLCs struggles to advance at the same rate as the rest of Western Europe, as the corruption of the Sejm and especially the Veto were a large factor in this and the states eventual demise.
An alternative to using the Sejm as directly as the age up mechanic would be to use the multi-ethnicity here. In this system, each age-up would be a joining with a different culture or state - if something like this is the case it is the defining feature of the civ. Essentially, with each age you would select not only an age up bonus, but a full roster of units, techs, and cards that come with it. As noted previously, I have not played much in the past couple years, but I believe this idea is fairly similar to the African Civ aging mechanic, which is why I think it is less likely. This would look something like Age2[Focus on Poland, Focus on Lithuania, Focus on Unity], Age3[Incorporate Cossacks, Incorporate Tartars, Ally with Hungary, Focus Internally], Age4[Elect King(Swedish, Prussian, Austrian, Russian, Polish/Lithuanian)], Age5[Choose Partitioner(Russia, Prussia, Austria, Rebel State)].
Although I don't think it is necessary in this case, deck mechanics seem to be a place where the developers are differentiating their new civs. I have no real ideas for a cool and unique thing here, but I think either Age Up mechanic I presented could easily be used to add cards to the deck in various unique ways. This general idea has, of course, already been done, so I doubt this will be the approach. I think a more likely thing would be to implement the multi-ethnic military of the PLC in the card options. A good approach to this would be to have a number of Home City unique units, so that they can only be produced through cards. These would be something like mercenary units, but not quite as much stronger than a normal unit. A number of infinite use cards to summon otherwise innacessible units would be an interesting military situation - and could be well supported if there are any XP generating mechanics in the civ.
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.
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).
The explorer unit will almost definitely be a Szlachcic (noble). This will need to also be a cavalry explorer, which would put the PLC in the top ranks of Age 1. I think a way to make this more interesting would be to have this be a non-hero unit. So it lacks the normal healing and revival powers of the normal explorer, but is also a trainable unit - maybe with a cap greater than 1.
The PLC was one of the last European states to have serfdom, and was one of the largest grain producers in Europe. This is something that could be represented in game. Others have speculated at having serfs as extra cheap villagers. 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 (again, outside of my actual good game knowledge). I think there will likely be some mechanic representing this general concept, and allowing the PLC to produce extra food, but I don't have any real thoughts on an implementation at this time. I suspect that the serfdom thing won't be represented, as even though it is memed on often. Russia had serfdom for much longer and that isn't really expressed through anything in game.
Others have noted that both new civs (PLC and Denmark) have a Dual Monarchy during the games time period. It would make sense, for thematic reasons, for the developers to include some sort of Dual Monarchy mechanic. This would reduce workload and give the expansion a unifying theme. I consider this to be highly likely. I do not, however, have any thoughts or ideas on what such a mechanic may be. The Age Up mechanics I presented both would include this theme for the PLC, but I don't think they are particularly transferable to Denmark, so there will be something else that unifies them together.
In conclusion, I suspect that the PLC will be a largely early game cavalry focused civ, but with varied late game options. I think that the heavy cavalry unit will be available in age 2, and be the most powerful age 2 cavalry unit - this will define the gameplay, as it will be a major advantage that needs to be capitalized on. The late game will not be powerful at its core, but I suspect there will be something that allows you a variety of gimmicky options to surprise your opponent with. So the core gameplay strategy will be to gain an early game advantage, and press that using a hard to respond to power spike later on.