I think Jon is coming back.
Sure -- GRRM could kill off Jon, and Dany, and Tryion, and admit that 75 percent of the first five books was just misdirection. If he really wanted to, he could then have Wonder Woman step out of a trans-dimensional time warp controlled by Egon Spengler. Forming an alliance with the Stay Puft Marshmallow Man, Wonder Woman harnesses the dragons, as it is simultaneously discovered the marshmallows are even more effective than dragonglass against White Walkers. The threat averted, Peter Venkman assumes the Iron Throne.
But assuming GRRM doesn't want to turn the first five books into a monumental waste of time and a joke against his readers, he has written himself into an endgame that needs to revolve around a rather limited number of characters. It's awfully late to be introducing a new major player. That would be deus ex machina stuff. Poor craftsmanship. The big three don't all have to survive to the very end, but it's much too early to waste them now.