You can mostly force countries to join by war now as the Emperor, with the Expand the Empire CB.
It’s better than nothing, I suppose, although it does feel meddlesome, takes your time away from conquering outside of Europe, and accumulates AE, strangely enough.
Attack a large country (France, Ottomans, Poland, Aragon...) make it release smaller states, wait out the truces, attack those smaller states and force them in.
And again. And again.
Edit: and hopefully, they’ll be small enough to both join the Empire and force-convert if wrong religion. Otherwise it’s a war #3 for the same clay.