According to the official wiki, any nation with Russian Principality government can turn into a Tsardom by forming Russia or Ruthenia. So all I need to do is to start as a East Slavic minor nation in 1444 and occupy Ukraine territory in order to activate the Forming Ruthenia decision.
But that's just not seemingly possible. All minor East Slavic nations in 1444 are either a vassal of Muscovy or too weak to conquer anyone. Even if we disregard the Muscovy problem, Ukraine territory is under Lithuanian occupation. I once tried to conquer Lithuanian land with Novgorod in 1450s and it was a very tough opponent. I don't think I can defeat Lithuania with nations like Odoyev even if Poland didn't form union with Lithuania.
I have formed Ruthenia with Poland-Lithuania Commonwealth by changing primary culture for some times. It was easy actually. But that Ruthenia is simply a common western kingdom with elective monarchy like Poland. Start as Muscovy and change culture into Ruthenia is the only way I can come up with, but that's not even slightly reasonable. Joining HRE with Catholic Ming is less weird than that.
But that's just not seemingly possible. All minor East Slavic nations in 1444 are either a vassal of Muscovy or too weak to conquer anyone. Even if we disregard the Muscovy problem, Ukraine territory is under Lithuanian occupation. I once tried to conquer Lithuanian land with Novgorod in 1450s and it was a very tough opponent. I don't think I can defeat Lithuania with nations like Odoyev even if Poland didn't form union with Lithuania.
I have formed Ruthenia with Poland-Lithuania Commonwealth by changing primary culture for some times. It was easy actually. But that Ruthenia is simply a common western kingdom with elective monarchy like Poland. Start as Muscovy and change culture into Ruthenia is the only way I can come up with, but that's not even slightly reasonable. Joining HRE with Catholic Ming is less weird than that.