For every Game of Thrones/A Song of Ice and Fire fan theory that hits the nail right on the head, it seems there's another that misses and breaks your hand instead.
R+L=J, the biggest fan theory in the history of the saga - both books and show - has now been confirmed in the latter, and it's only a matter of time before it is canon in the former as well. It was a perfectly formed theory, picking up on the various clues throughout the text, and one that pretty much everyone believed.
On the flipside you've got the 'crackpot' or 'tinfoil' theories, such as Varys being a merman, Roose Bolton being a skin-stealing immortal, and Gendry actually appearing on the show again.
They're a huge part of being a Game of Thrones fan, but with the biggest one now considered true, what's left? Well, quite a lot actually. Ned Stark is still alive, so is Robb, and literally everyone is a Targaryen.
In terms of those that are actually believable, there's much more than just R+L=J. Season 6, for example, went around the broadstrokes of the Grand Northern Conspiracy theory, and went to great pains to setup and then dash hopes of Cleganebowl.
Excluding the ones mentioned, then, and with an attempt to keep it to those that could still work in book and show (with a couple of exceptions), these are the fan theories that, while pretty out there, could actually be true.
NCTJ-qualified journalist. Most definitely not a racing driver. Drink too much tea; eat too much peanut butter; watch too much TV. Sadly only the latter paying off so far.
A mix of wise-old man in a young man's body with a child-like wonder about him and a great otherworldly sensibility.