Prior to 1964, the Catholic Church had Latin rite Patriarchs of Alexandria, Antioch, Constantinople and Jerusalem. The former three have been abolished, while Jerusalem remains (as there's a non-trivial number of Latin rite Catholics thereabouts). Your challenge is to conjure a scenario where Moscow is added to those ranks. The Patriarchate of Moscow can be titular or have a substantial flock; the latter would be more fun than a "Polish rule lasts 20 years, Rome creates the Patriarchate to try and sway the locals, then it all falls apart and only the office remains" scenario, IMO. Have at it!