Agreed. Well, that particular promise - the exact content of which has never been revealed - seems to have haunted (and guided) Ned for the rest of his life. Again, I'd be most surprised if it didn't in some way concern Jon. And, given that, I would be further surprised if some strange other-worldly device did not return Jon in some way for his tryst with destiny later in the series. Neither Jon's backstory nor his future (unlike every other character - the ones killed invariably had a full backstory) have been sketched in more than the merest of detail, a strange omission for a character to whom so much space and so much emotional investment has been made. I expect to see more of him in some capacity.