I'm posting this on behalf of the original writer (corey) bc he is having issues getting his response to post.

"I think the Targ's resistance to fire is fickle at best, but in this case will be used as a means to prove his heritage. I should also clarify that I don't believe they will stick him in the fire and oh no he doesn't burn. I believe like how Miri Maz Dur was involved in Dany's resistance to her burning so will the Red Woman be involved in some means with Jon's. I don't think it's outside the realm of possibility to say Davos might not be exactly happy with the Red Woman. You burned Shireen? I'm burning you. Something along those lines. Also a possibility is some type of love sacrifice ala Khal Drogo and Dany. Maybe Ghost is substituted? Again, I'm not saying the details are locked down, but simply some type of burning of Jon's body will reveal his heritage. Also, I understand there are some that could give their word to Jon's true heritage, but if you want people to believe, you have to have some sort of PHYSICAL evidence or event. People arent simply going to flock to Jon because Howland Reed says so. They will need proof. Jon's inability to burn, however it is accomplished, would prove this. And lastly, in the books Bran has a vision of Jon going cold in a cell of ice, or something along those lines, which could easily be explained by Jon's inability to burn. If he can't burn, it's not unlikely the NW's sticks his body in a cell while they try to figure out what the hell to do with him, and if his spirit warged into Ghost, he would "forget what it was like to be warm" as stated in the books. If Bran has these visions, and is becoming a Jedi Warg Master with Bloodraven, not impossible to assume Bran helps Jon back into his body somehow.

I know that Viserys was burned to death, but again I am saying under the right circumstances Targs can be immune to fire. Magic is a fickle b***h according to GRRM, First of His Name, and does not always work in the same manner for every person. So Jon may not have total immunity to fire, just how even Dany does not, her hair is burnt off in the books. But with some sort of help from the Red Woman, Ghost, Bran, etc, his body could survive. And I don't buy for a second that Jon will rise again in someone else's body. His body/DNA is unique due to the combo of Stark/Targ, and he would lose it if he warged into Hodor or something like that. Also of note is how Mel discovers her 'powers' are amplified at the Wall in the books. I understand these things have not been pointed out on the show, but they give D&D a blueprint for Jon's resurrection."