[Toy Soldier]

I read a theory that

Spoiler:

Jon will ride the Wight/Ice Dragon. With the dragons connection to the targaryen bloodline extending beyond death, Jon will manage to take control of it in battle and it will turn on the walkers. Apparently there's been a theory about an ice dragon floating about for ages; some speculating that there might be one frozen inside the wall... But it'll now probably be viseryon.

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It does make sense for the "song of ice and fire", with Jon being g "ice".