Also, if they ever fix and rework Russia-Ruthenia-Poland-Steppe region again, I think that adding a monument to Kiev province is in order - Kyiv Pechersk Lavra (
https://en.wikipedia.org/wiki/Kyiv_Pechersk_Lavra), one of the first orthodox monasteries founded in Rus' after covnerting to Christianity and important site for the piligrimage. Up during the start of EU IV date it was sacked by armies of russian princes during their countless feuds in XII-XIII centuries and later on by the nomads, so it should be lvl. 0 i think, lvl. 1 at max (Moscow Kremlin should be lvl 1 i think).
Not sure what bonuses it should give, though. Considering that monastery served as a shelter for those who resisted Brest Church Union, maybe it should give Kiev province "Religious center" modifier so Lithuania and PLC will find it harder to convert into Catholicism? As it was discussed in some threads long before, the poor state of Ruthenian provinces under control of Lithuania makes it so much easier to convert and there's no representation of resistance to that at all. But again, that would require a complete revisit of the whole area, maybe "balkanizing" Lithuania a bit with vassal princedoms (adding special interactions, disasters with them, etc.) and a rework for stepee Hordes...
A little offtop, but nevertheless: maybe devs would consider adding loading screens with Russian Tsar Ivan IV Rurikovich (personally, I prefer the "nickname" Fearsome over Terrible) and Polish King Stephan Bathory? Both were rivals in the respected time period and all played important role in the history of both countries, plus Stephan gives a nice little hint to Transylvania, which wasn't present in any other loading screen before.