I've currently returned to my game as the King of Pomerania. Started in I think the early 1120s with the first Christian member of House Gryfita (aka House Gryf or the House of Griffins) who historically would go on to be the first kings of the mighty Kalmar Union (and thus one of the most prominent starting houses in EUIII!), snatched up enough de jure land to form the kingdom, and converted an eventual ruler to the ol' Romuvan faith. Promptly swore fealty to the HRE and used my time there collect the rest of "de jure" Pomerania, while also using their protection to safely pursue incursions into Lithuania. Re-converted before all my stuff got revoked, and now I'm once again independent, thus saving the slavic Old Pomeranians from their real life fate of eventually culturally assimilating into Germans.
Its been a fun game, though sadly my time in the HRE has resulted in a few unfortunate borders. Thanks to my AI family and vassals, I also hold a single, random county in the middle of Germany (Leiningen), half of Holstein (the two counties that DON'T border my de jure kingdom), the INLAND half of the Duchy of Verona, and the entire Duchy of Sicily... which would be cool, but the Duke of Verona inherited it and has refused to move his capital there, so all his troops pop up in the middle of the Alps. Dick.
But personally, I've carved out all of de jure Pomerania for myself, along with just over half of Lithuania with the exception of the Duchies of Livonia Polotsk, which is completely in Sweden's and an independent Rurik's respective hands. Just taking one great dynasty and giving them a new destiny is all.
...
Guess I should fight the (also independent) King of Lotharingia for Holstein. Didn't really want Holstein, but I guess I if I took the rest of its de jure lands it'd at least be connected... unlike Leinigen, Verona, and Sicily...
EDIT: Bonus note about House Gryf: the ruler of Pommerania at the start of EUIII's timeline, was a bonafide
pirate. House Gryf: rulers of Pomerania, Scandinavia, and
freaking pirates. (well, one of them was... outside of CKII, sadly...)