I'm pretty bad about starting to write TLs and then abandoning them. I had one where Edward IV is healthier and lives longer, with an ATL Reformation happening about 30-40 years after POD. Protestantism does pretty well in many regions of the Empire, and a dispute over the appointment of bishops sparks a break with Rome.
Sounds interesting, though how exactly England is able to influence the rest of Europe at that point in time has always puzzled me a bit. After the 100YW and the Wars of the Roses and into Queen Elizabeth's reign England was something of a second-rate power particularly when compared to the HRE, Spain or France. That is why a PoD in continental Europe would probably make more sense. Personally I lean towards something in the pre-Peace of Augsburg Era either related to the Italian Wars, Wars of Suleiman or directly with the Reformation if this should have a chance of working. You really need to work with the period before Lutheranism, Reformed and Radical Reformations start finding their final forms, and the same with Catholicism. Intervention either in the pre-Counter-Reformation or during the Counter-Reformation would probably also be important if something like a Protestant Emperor is to work.
Of the seven electors, three were Catholic archbishops and the other four were secular rulers. Three of the princes were Protestants but the fourth, the King of Bohemia, was a Habsburg and thus Catholic. Bohemia needs to break free from Habsburg rule - and stay that way - for a Protestant to win the election. IOTL when it broke free, it triggered the Thirty Years' War. Maybe with an earlier POD it is possible?
The other possibility would be for the Habsburgs themselves to convert, although that is tough.
As mentioned, I think that a pre-Peace of Augsburg PoD would work best for this. I was wondering what the likelihood of Albrecht von Hohenzollern, Elector of Mainz, providing support for a candidate in return for secularizing the Archbishopric. The Elector of Cologne also expressed significant pro-Reform sentiments in the pre-1540s crisis period, so there should be plenty of opportunitites to turn the Archbishoprics in favor of a Protestant Emperor if you time it correctly.
If I recall correctly, the proposed imperial diets gave the electors some sort of elevated status in politics, but I forget the exact details. But yeah, as you said, the question is what circumstances could allow for the emperor to keep the diet around.
I'm toying around with this concept of a more successful 1495 reichsreform ATL - basically the goal is to create a semi-federal HRE (kinda like OTL modern Spain) based around the idea of imperial power being shared and balanced between the princes and emperor.
You need to keep in mind that the princes are by no means equal, and that there is a layman-clerical division between them. There are hundreds of petty duchies who would be insulted to find themselves relegated below the Duke of Bavaria or the like, but at the same time the larger Dukes would be unwilling to share power equally with the petty dukes. At the same time you have the Electors who would view this as an attack on their priveleged position unless they receive special extra powers of one kind or another.
In the second half of 16th century, Habsburgs were not particularly zealous Catholics. Rudolf II, Matthias...
If either of them outright converted to Protestantism, who´d overthrow him?
Rudolf and Matthias both faced significant opposition from within their family, and publicly converting would have served as a perfect excuse for their own rivals to attack them and their supporters to turn on them. That said, particularly Rudolf II has quite a lot of influence from the Spirituali movement in Italy during the 1520s and 1530s with many of the reformist ideas being considered for adaptation by the wider Church, although after the 1541 crisis the Spirituali were driven into exile and joined the Reformers, particularly Jean Calvin, in Geneva or Strassburg.
Actually, the Bohemian vote isn't necessary (although it's the easiest way to get a fourth Protestant Elector). The three Archbishops were chosen not by the Pope but by the local Cathedral chapter, and thus weren't strictly required to be Catholic. There was a point at which the Archbishop of Cologne was actually a Protestant, although he was quickly deposed in the Cologne War of 1583-1588.
Reading up on it, Archbishop Gebhard converted to Protestantism after his election by the Cologne Cathedral chapter. Also, the Peace of Augsburg had stipulated that any Prince-Bishop or Prince-Archbishop who converted to Protestantism would thereby resign his post. So, we might need a POD before the Peace of Augsburg (1555) to have a Protestant recognized as Archbishop-Elector.
While the Cologne War and Archbishop Gebhard could work very well, there were also several earlier bishop-electors who could work as mentioned above.
That said, I agree that a pre-Peace of Augsburg PoD would definitely work best for this.
Cologne War did last 6 years.
So WI Gebhard wins Cologne War? Would that make Peace of Augsburg a dead letter?
I am unsure fo the effects of a victorious Cologne War for Gebhard, but it could well provoke a Empire-wide war. Could be an interesting PoD...