He owns a valyrian steel sword, Heartsbane. To forge Lightbringer, you have to sacrifice the person you love most. If Sam were to kill Gilly with it for some plot driven reason, it could make him Azor Ahai. Underdog, unexpected twist and all that etc.
Jorah is my real gamble for it. In the previews, Jorah appears to be welding Sam's sword in battle. Maybe Dany does something that forces everyone to turn on her and Jorah has no choice but to swing. He loves her, so him striking her through the heart could do it as well.
I feel like Jon is too obvious. GRRM wouldn't put all of his eggs in one basket, I don't think.