Game Of Thrones Theory: Bran Wargs Into Night King's Dragon In Season 8
3. The Dragon Has Three Heads
Okay, this point is getting into some wilder speculation territory, but it is worth considering. When it comes to dragons, one of the biggest pieces of prophecy we hear in the books is the line 'the dragon has three heads'. This comes during Daenerys' visions in the House of the Undying, with the prophecy made by Rhaegar. And since so much of that has come to pass, it's likely that there is a real meaning to this too.
While the phrase didn't appear on the show, it's one of the book moments that has achieved some cross-medium recognition, and the line underpins a number of theories about who will ride a dragon and who might be a secret Targaryen, which has largely focused on Tyrion, who it just so happens has shown a certain affinity with the dragons.
However, if we're talking Targaryens, then there has already been another one on the show... sort of. The Three-Eyed Raven is widely believed to have once been Brynden Rivers, a bastard of King Aegon IV Targaryen who, along with many others, was legitimised on the King's deathbed. Brynden, who was also known as Bloodraven, fought in the Blackfyre Rebellion, before taking the black and eventually becoming Lord Commander of the Night's Watch.
It's not explicitly stated that they are one and the same, but there's enough that it makes sense and can be taken as canon. So with this stretch, then, we have the Three-Eyed Raven as a Targaryen. Now, Bran is the Three-Eyed Raven - in a more symbolic, title way than a literal same person sense - which gives him a link to the Targaryens, and means he can fit with the idea that the dragon has three heads by warging into Viserion, with Dany obviously on Drogon and Jon very likely to ride Rhaegal during Season 8.
This is further alluded to by the key line the Three-Eyed Raven says to Bran at the end of Season 4, upon their first meeting: "You will never walk again, but you will fly!' Although this does refer to him becoming the Raven and, later, literally warging into a raven, it could well apply to him warging into a dragon as well.