I have successfully dismantled the HRE for the first time in EU4:
It was only by the thinnest of margins, as evidenced by the size of my armies on the outliner. I declared war on Austria in 1774, with a goal of taking Munich as well as surrounding lands with the aim of establishing Germany. That initially succeeded, although as seen eventually I lost Munich and Croatia (my ally, whom I freed from Hungary a few decades prior) occupied it. But I realized that of the six electors (one had been conquered), I was already occupying The Palatinate, and if I declared on Nuremburg, I'd be at war with the other five electors as well, including Bohemia who was the Emperor. Alas, Hungary also would fight on Bohemia's side.
So I started a second war, with no allies save my vassal Poland in that war. I intentionally placed troops such that within two months, all save Augsburg and Bohemia (level 4 forts) and Hungary were defeated. But Bohemia and Hungary have about 70,000 troops just on the screen, and a few more not visible; they likely could have defeated my armies had they broken through my outdated fort in Ratibor, and then I would have had no hope save mercenaries of dismantling the HRE that war.
All in all, my armies went from 175K with 156K manpower in reserves to 37K. But I gained many provinces, forced Austria to release a few nations, took a couple of my rivals' strongest forts, dismantled the HRE, and got 92.5% overextension. The amazing thing is that France hasn't declared war on me; I do have strong forts on their borders that would slow them down, but this would be the perfect chance for them to defeat me, and while you can't tell it from this screenshot, they are a worthy rival and nearly equal in military prowess.
Future plans involve forming Germany, attempting to vassalize several small states in southern Germany/northern Italy, potentially experiencing a Revolutionary country, and potentially seeing if I can help the Ottomans recover from grabbing defeat from the jaws of victory - they've been imploding in slow motion for decades, but did finish westernizing. Currently they're losing to Ragusa, but that's their only war, and it would be a challenge to restore them to the better part of their former glory. A century and a half ago an alliance with them was my preferred route to defeat France; now it's questionable whether they can defeat an OPM.
This game is still on 1.12 without Art of War, Common Sense, or El Dorado, but I also have some multi-player games in progress with 1.13 and all of those expansions.
Edit: The Ottomans just became an Orthodox Republic. Mind = blown. It's quite possibly the best thing that's happened to them in decades nonetheless.