I found myself at the wiki page of Barad Gúlaran and this lore bit quoted there completely escaped my notice before:

"Unbeknownst to all but the Wise, however, he also built a fortress amid the wastes of Angmar, which he called Barad Gúlaran. There, he continued his studies in the dark arts he had learned from the Dark Lord Sauron.Though many secrets of power and the world of shadow had been laid bare to him when his Ring consumed him, there was much still he wished to learn. Chief and most secret of these was a way to break the bonds that enslaved him to the Lord of Barad-dûr, and thus make himself a great Power. When Eärnur, Prince of Gondor, and Glorfindel of Rivendell drove him from Angmar, the Witch-king’s hopes of gaining this power were dashed. He could never again return to Angmar, and was forced to leave his secrets in Barad Gúlaran free to whoever sought them." — lotro.com

Immediately made me think of that Máltagar relic introduced in Gundabad, maybe that's connected somehow. It's a cool idea that a Nazgul as powerful as the Witch-king would contemplate his freedom - I guess maybe that was his only shot at it... since Sauron wasn't at his best, while diminished and without the ring, plus Angmar was far away enough for WK to be a little secretive? His Angmar defeat would have hurt ten times more with that context in mind plus his later obsession with tethering of Earnur to himself just as he himself was tethered to Sauron... well, this could be pretty deep, actually.. never thought I could find something so deep in a Lord of the Nazgul but here we are