He'll die. Theon has had one of the more depressing character arcs on this series, so our fingers are crossed that at least he'll die an admirable death while trying to protect Bran. He's not a good fighter and he's earned a reputation as a coward (though, to be fair, he's been suffering from an extreme amount of PTSD thanks to Ramsay Bolton), but as we see earlier in the season when he rescues his sister Yara, Theon is determined to redeem himself. Sacrificing himself for the greater good will do just that.