Another fan theory that's probably not true but makes a whole lot of sense is that Roose Bolton is a vampire. Sure, we know season six unfolds with Ramsay Bolton stabbing his father in the chest, but that doesn't totally dispel this notion. After all, the dagger could have contained Valyrian steel, which is known to kill almost everything.
As outlined by Reddit user Megatron_McLargeHuge, most of the support for this theory comes from the series of novels. In the first book, George RR Martin describes Roose as follows: "His eyes were curiously pale, almost without color, and his look disturbing." There's also the fact that the Roose Bolton of the novels regularly takes leeches because "Frequent leechings are the secret of a long life. A man must purge himself of bad blood." The text also mentions that he barely eats and that, when he does consume food, he's known to use bread to sop up the blood from his rare meat. Moreover, Roose Bolton's sigil in the books and on the show is the flayed man. Some fans pinpoint this as an indicator that Roose needs to skin his enemies and wear their human pelts as disguises to conceal his eternal life.
Not to mention, the show has featured every mythical creature imaginable — dragons, zombies, werewolves (direwolves), witches, and even Qyburn's monstrous resurrection of the Mountain feels Frankenstein-esque. Why not a vampire? Obviously, Roose isn't a traditional version of the creature because we see him in daylight, but the threads are there for those fans who want to believe.