How much land did the various princes and lesser lords actually own, and how much of their resources depended on revenues and the like?
You don't have to beggar the nobles to break them - just defeat them militarily, then make them dance attendance at the imperial court. Louis XIV perfected it, but French kings had been doing it for a long time, and the combination was how they cut the duchies down to size. So I imagine that the Empire might remain outwardly a clutter of princely states, free cities, etc., but with a drastic reduction of 'states' rights,' and development of a grand Imperial court (not just for vanity but control), an imperial army, and the first stages of an imperial bureaucracy. The overall impression, by the 17th century, would be broadly comparable to France, a grand monarchy.
If the Empire goes Catholic, Protestantism is probably over, except as an underground movement. Even if Scandinavia and England are Protestant, will they cut themselves off long term from the rest of Western Christendom? The Church of England could easily drift back to Rome, if the whole thing isn't butterflied to begin with.
If the Empire goes Protestant, it could get weird. Henri IV doesn't need a Mass to get Paris, if Imperial support hands it to him - again if this is not all already butterflied. But what happens if a proto-Protestant Emperor rolls into Rome about 1530? Protestantism does not quite exist yet in 1530, and remember what Reformation originally meant. He muscles a reformist Pope onto St. Pete's chair, who calls an Ecumenical Council to clean up the mess and clarify the rules.
The religious end result in our time could be a Catholic Church that looks institutionally and theologically rather 'Anglican.' The Pope perhaps remains presiding bishop of Western Christendom, but final authority resides in a general council - or perhaps more likely in this scenario, the Pope becomes a de facto imperial appointment. The English monarchy from Elizabeth on did not have more than nuisance trouble from the Church of England.
Theology? Above my pay grade, but if this is a Protestant or at least proto-Protestant win, the Mass becomes a holy communion, and 'faith alone' becomes church doctrine. I have to wonder if 'scripture alone' doesn't get modified by people who find themselves in control of an ecumenical church, in a position to promulgate doctrine and a magisterium to teach it.
The fun thing is, the farther back you push the POD - or not so much the POD itself, but the proliferation of jumbo butterflies - the more the Catholic and Protestant outcomes blur into each other, finally becoming a sort of Erasmian reform.
This doesn't even touch on the political consequences. In broadest brush, the French monarchy is no longer the biggest kid on the European bloc whenever it has its domestic act together. The mostly German Imperial kid is at least as big, or somewhat bigger.
Somehow I get the picture of subsequent European history as an incredibly slo-mo version of World War I. Germany and France can't conquer each other, and given available military techs and doctrines, the likely outcome is great chains of fortresses, a 17th century Western Front. I don't mean a 300 Years' War, but however many wars through the 16th and 17th centuries that eventually all wear down to sitzkrieg.
