PoD - Ivan the Terrible takes the PLC crown. The Union becomes constitutionally Real, not Just Personal.
Between 1570es and 1640es they conquer Siberia, Manchuria ( possibly China! ) and reach Alaska, California.
During the Thirty years war they side the Catholic league. Conquer all of Scandinavia.
Since 1648 the PLMC Emperor becomes Holy Roman Emperor. Crowned by collegium /Sinode of the 5 Patriarchs. All the time ongoing hardly enforced project is restoring the unity of the Church i.e. re-merger of the Catholic and Ortodox churches.
This *HRE consisting of OTL HRE, Swiss, Italy, PLMC ... ( the backbone is pan-slavic. Italy and Germany and Scandinavia are the western periphery, which is going to seccede at some future point and turn into mega-*Switzerland - neutral buffer zone; and the *HRE Church re-merger policies make the Atlantic European countries to revert to "protestanism". Spain+Portugal, France, England+Scotland+Netherlands furnish themselves with the equivalent of "Anglican churches". The Atlantic European powers rush into the Oceans even more ruthelessly.
The Atlantic euro-powers and *HRE expansion waves colide in North America on Mississippi, in Africa on Congo river, in Asia - in the Malayan archipelago.
Australia, New Zealand, Oceania are *HRE-ed cause they are closer and more convenient from Dalien, Manchiria.
Around the end of 17th century the Ottoman empire ( and generally Islam ) is crushed and absorbed into series of Hyper-crussade. In result no territory ITTL has majority muslim population.
Between end of 17th century and mid-19th century India is consolidated under *HRE.
*HRE is trully H because it holds the "true christianity" and truly R cause it holds both Rome and Constantinople.
The *HRE official language is artificial Slavic language like Slovio.