Well fire is Dany, and Sea would be Euron/Victarion, so its not that crazy. The only problem is I don't think Jon will be a dragon rider, so I'm not sure who winter would be. I still lean towards Tyrion as the third dragon rider, who doesn't seem to represent any element.
I just wanted to add, that it seemed clear to me that Brienne didn't kill him. People are assuming she did, but I think it's a fake out. Which would make sense if Stannis is supposed to lead the fight against the Others, because back to the wall might be his only option.
It's probably where the story is going, but I think it's kind of a cheap loophole. The oath basically means you are in the NW until you no longer exist. If your heart stops beating, and then you are revived, that doesn't count. I feel like Jon is too honorable to use that loophole, unless he has a really good reason.
Maybe as Ghost he'll see something in the lands of always winter, and it'll give him reason to try and get out of his vows.