Gustav II Adolf toyed with the idea of declaring himself protestant Emperor. What if he had lived, seen a good victory at Lützen, had an heir and declared himself Emperor?
Sweden is too poor and desolate to keep such a title for too long, it will pass to the Prussian King sooner or later, methinks
Couple of problems.
One, nobody but Sweden likes the idea of a Swedish Protestant Emperor. Ferdinand of Austria will hate it because it's a rival Emperor. Max of Bavaria and the other Catholic German princes will hate it because it's a Protestant Emperor. Even the Protestant Germans will hate it because it's being done by a foreigner, and at sword's point. (The Saxons would probably use it as an excuse to bolt the Protestant coalition; they were edging towards the door OTL.)
The French will applaud -- anything that keeps the war in Germany going is gravy -- but they'll also back offo; by Lutzen, they were already getting much less supportive of Gustavus. He was a bit too successful, and not willing to be a docile stalking horse for his French paymasters. So, they won't give much tangible support.
Assuming Gustavus lives, he's going to be in some serious trouble. Yes, he was a brilliant general and administrator, but he's going to be running out of money and allies tout suite.
Two, there's no plausible prospect of settling a war once a second Emperor is declared. It took more than a decade after Lutzen to settle OTL! TTL... well, if Ferdinand was willing, in order to reclaim the title of King of Bohemia, to see his Bohemian kingdom utterly devastated, what do you think he'd do to reclaim the Imperium? He and the Hapsburgs would never, ever accept a rival Protestant Emperor.
Three, forget about Prussia. Brandenburg was one of the feeblest major states in Germany. It had the dignity of an Electoral title, but otherwise it was negligible. Berlin was a modest country market town, and the Brandenburg army was a joke. Prussia's rise wouldn't begin for another couple of generations.
Mind, I don't think a Protestant Emperor is a completely insane idea. But I don't think the 1630s were the time to do it, and I don't think Gustavus was the man.
Doug M.