Polities joining the Empire should be rare. I know that in EUIV it's possible to make the HRE a much more centralised entity, but historically only East Frisia, Flanders and Artois joined the HRE after 1444. Hungary was never even incorporated.
Polities joining the HRE, or refusing to, should make sense to the player and ideally to the AI.
Remind me again how the game treats threats. After all, in a test run I punched Krakow out of Poland, got their relations to 200 ... and they refused to believe that Poland, with permacores and all was a bigger threat than Bohemia. Fine. I console integrated Bohemia. Now they think I, as Austria, am their biggest threat. How is that rational? Both Poland and I were truced with them, and I had just released them in war and improved relations. And lest we forget, Hungary had marked them as a strategic province. In all directions they faced states that could roll them in a single war, they had no sufficiently strong allies, and our relations were great.
Or take another test run. I play the Ottomans. I console borders to Croatia and mark their territory. I scornfully insult them. And the console says that they are threatened. All Austria (who has declared me a rival) has to do is improve their opinion of Croatia and they can deny me territory and strengthen the empire … and they do nothing for 10 years on speed 5.
The game has utterly absurd power dynamics for the Emperor who is utterly incapable of expanding the Empire even if they want to. Features should not be Human-Only whenever possible.
Ultimately, we don't get rare expansion. We get a paywalled CB that lets the emperor (and only the emperor) expand like mad for the cost of a single war, a hodgepodge of crapshoots for relations/most threatening enemy, and incidents. We either get virtually no expansion or we get scads. Further we get no mechanism for the AI to oppose expansion other than the tired formulations of AE/coalition. Nowhere do the Turks do anything to keep Bosnia out of the HRE. If they have a falling out with Ragusa they do nothing during the truce to prevent them from going into the HRE.
The HRE could do far better and could be far more strategic than it is now.