So I have Expand Empire CB, which I can use to make countries join - and that gives me some free IA.
I got Moldova, Ferrara, Savoy, and Naxos. And while at it I made some of their allies release a bunch of OPMs like Corsica, Lucca, Parma, Donauworth, Bamberg.
I wanted to have a glorious fight against Spain for Naples cores on Sicily, but it turns out they expired, as Paradox split Neapolitan and Sicilian cultures. Those silly microcultures are seriously annoying.
Well, what else can I do? Turns out I can force Denmark to join the HRE. I didn't expect that. France which I recently allied was on their side, but everybody in Europe really hated Denmark, so I had a huge alliance on my side - all the while France was busy fighting Spain.
I made France and Pope release some minors, and got Denmark to join the Empire. A small problem is that they're heretics, but I'll get to it eventually. Not sure how, as Denmark -200 hates me, and even if they loved me they're enormous, but opportunities show up every now and then, and heretic penalty is per country, so they only cost me as much as Ulm would.
Than unfortunately only added mainland of Denmark, not Norway or Livonia, so if I force them to release Norway or Livonia, they'll leave the Empire. Quite annoying.
That costs 25% AE cost, the same as returning cores, so it wasn't quite free, but I managed to convince everyone to please not coalition me just yet.
After that war, I passed the 4th reform. It's going to be about 15 years between reforms by just ticking IA, so counting all other sources I'll be able to revoke about 1650-1670.
Once our 15 year truces expire, I plan to add about 5 more countries I recently released to the HRE, if they're still around. It would be great if I somehow managed to fragment France so much that what's left can be added, but I'm not sure if I can make it.
Playing as Austria, and having all those CBs by 1470s would be really powerful - this late they're just less amazing.
Oh and there's supposedly some way to make countries join peacefully, but I have no idea how that even works.