The problem with panslavism is that Russians would have majority, and Russian states were almost always authoritharian and oppressive.
Almost all former European monarchies, from England to Belgium, were authoritarian imperialistic countries.
But even compared to the absolutist monarchies of the west Russia was more authoritarian. For example serfdom was only abolished in Russia in 1861.
Russia wasn't always authoritarian, though. Before the Mongols, it was a bunch of independent, freedom-loving cities. Sort of like the American Wild West.
I feel like we need a new experiment in inclusive governance, and that Slavic countries are the place to do it.
Grand Duchy of Lithuania is a better model, IMO - a state with a Lithuanian pagan ruler, and a mix of pagan, Catholic, and Orthodox elites and population all living together.
(1) The Polish-Lithuanian Commonwealth had elected kings and a parliament. It was a republic to the same degree as Rome or some of the more democratic Greek city-states. The only people who could vote were nobles, but nobles were about 10% of the population. That's about on-par with with Athens or the Roman Empire. Even in the US, only white landowners could vote when it was first founded. That's not too dissimilar from the Polish nobility.
(2) Jews emigrated to the Commonwealth from all over Europe for better treatment. Good treatment? No. Better treatment? Without a doubt.
The Commonwealth wasn't a modern enlightened democracy, but this was 1600, not 1900. It did incredibly well for 1600.
For one thing nobles took a lot of Jewish immigrants, used them as middle management (which frustrated the church, who in turn sent angry peasant mobs against Jews, which meant Jews had to work with nobility for their protection).
Also a lot of nobles weren't Catholic, especially in 1500s protestantism was very popular (about 1/3rd of nobles and majority of magnates were protestant). Only in times of Zygmunt 3 Waza who was super Catholic and introduced some discrimination it changed significantly.
In fact nobility forced kings to sign https://en.wikipedia.org/wiki/Warsaw_Confederation which banned religious persecution.
Majority of nobles from eastern parts of PLC were orthodox at first (later they polonized and switched to Catholicism mostly), and there were some Jewish and Muslim nobles (but it was very rare). They still had all the rights including voting for kings, wars and taxes.
> There's a reason why there were so many uprisings on the territory that is Ukraine today, back when it was a part of the Commonwealth
Mostly economic, but yes, there has been some religious persecution there. Especially after the attempts to create orthodox-style pope-sanctioned religion in Ukraine.
As for nobles not carrying out religious persecution, I don't even know what to say. Local magnates were the main drivers of the religious persecution of Orthodox Ukrainians and Belarusians. And sure, that persecution was often economic in nature, but it was also very clearly carried out along religious lines.
I'll grant you that religious intolerance in Poland was primarily between Catholics and Orthodox, rather than Catholics and Protestants as elsewhere in Europe. But that's to be expected from a country where most subjects are either Catholic or Orthodox, no?
EDIT: I suppose our perspectives are so different because we're focusing on different time periods. My take on this is that early Polish-Lithuanian Commonwealth inherited many of the political arrangements of GDL, and tolerance was one of them. But as the center of political power shifted over to Poland over time, they were weakened.
There was some of that (especially in lat 1600s), but compared to basically any other country at the time it was significantly better. PLC even had a law that each newly elected king had to sign where he promised not to persecute anybody because of religion or he stops being the king: https://en.wikipedia.org/wiki/Warsaw_Confederation
Majority of magnates (the big nobility that actually decided things by buying other nobles during elections) weren't Catholic in 1500s.
Austria should be replaced by Ukraine and Moldova, though. Austria definitely doesn't belong.
Ukraine and Poland really do belong together. Ukraine is more faithful to traditional Polish culture than Poland. Poland westernized a lot since joining the EU and NATO, and Ukranian cultural influence would be awesome. Conversely, Ukraine could use some economic assistance, where Poland's economy has exploded, for much the same reason of NATO/EU.
Yet, this idea would be dangerous even with very different cultures - vide "Ein Reich".