The title is somewhat flippant, but I'm starting to branch out and read about Scandinavian, Russian, and Polish history.
One thing that's striking me, given my cursory knowledge of the period, is how integrated the states were and how active Poland was. Unfortunately, Sigismund was a devout Catholic, while Sweden was hardly a land of ecumenical tolerance at this point. [1]
And then of course, in the Russian Time of troubles, Sigismund's son Władysław IV Vasa was proposed as Tsar (offering to convert to Orthodoxy), before Sigismund decided the crown would sit better upon his head. As a Catholic Tsar. [2]
I think the idea of an ecumenical family compact of Sweden, Poland-Lithuania, and Russia, ruling based on religious tolerance, intervening in the Thirty years War is sadly implausible. (Though may worth sketching out, just for Susano's reaction). But it seems to me that there is a great room for play in this region.
Thoughts?
[1] I don't think we should forget that a lot of Vasa's "evil" actions included opening up schools for Catholics and appointing Catholics to high posts.
[2] The Russian reaction is known as the rise of the Romanov dynasty.