As a Polish person myself (although I'm not sure how representative I am

) I'd be perfectly fine with Poland not being a staple, in general the less staples the series has the better as it makes rosters more unpredictable and flexible. Poland is just one of many Eastern European nations overlooked by the popular history as they have never been hegemonic great powers and just because it is, let's say, by far the biggest economy in the region today, it doesn't mean it always has to take the place of one more Czech, Czechoslovak, Hungarian, Ukrainian, Lithuanian, Romanian, Bulgarian, Croatian, Serbian, Bosnian, Albanian etc civilization. In the high medieval era Bohemia and Hungary were far more prominent, in the early medieval era - Bulgaria was mighty, Ukraine has a ton of potential for unique and exotic civilization, Skanderbeg was hell of a leader, Romania actually can represent 19th century as it was the only one remaining (mostly) independent in this era etc. And you can also try aiming at more obscure Slavic civilizations of very early era.
(edit, I have just realized I started talking about "Eastern Europe" instead of "Slavs", but well you get the idea, Slavic and non - Slavic peoples are quite culturally close and inhabit the same region, sharing the same "set of slots" in civ series)