Conan the Barbarian/WMG

The Arnold movies can be reconciled with the original Howard stories
...despite the massive changes to everything. They simply aren't about Conan of Cimmeria, but rather Konahn of Simaria.

The sleeping god from Conan the Destroyer was really supposed to be Cthulhu
They refer to it as the sleeping or dreaming god constantly, and when his summoning is messed up he looks like a weird Cthulhu star-spawn. It would have been Cthulhu if she had been sacrificed but instead they got that thing.
 * This is acknowledged. Lovecraft and Howard were fans of one another's work. There's even an article of this here: http://www.cracked.com/article_19323_6-movie-tv-universes-that-overlap-in-mind-blowing-ways_p2.html. They published in the same magazines and put references to each other in their respective works.

When Conan attained royal status he took the name Kalidor.
Kalidor in Red Sonja is Conan after he became a prince and took Kalidor as his regal christening.

Valeria has a sister
Providing Kalidor is Conan, then the resemblance Queen Gedren has to Valeria is no coincidence; they are sisters, with Gedren likely serving the role of Evil Twin.

Valeria’s funeral pyre
When they first meet, the wizard Akiro tells Conan that his home is looked after by powerful gods and he later says great kings are buried in the mounds nearby and that “fire won’t burn there.” During Valeria’s funeral, he likewise tells this to Subotai, who proceeds to light her pyre successfully anyway.

This happens for one or two reasons: Either the spirits (the kings in particular) admire Valeria’s valor and so allow the fire to burn OR they permit it because Valeria is, in fact, of royal blood like them and they are embracing her. See the preceding theory for further supporting evidence.

Thulsa Doom is an Immortal from Highlander.
He's said to be immortal, and Conan kills him by cutting his head off! What more do you want?
 * When an Immortal gets his head lopped off in the Highlander series, there's a serious show involving lots of lightning going off as the Immortal's killer takes his power. There was no such scene in the movie. Most likely, Thulsa Doom was just very long-lived, either due to his Atlantean heritage or because he was a sorcerer. In either case, he could still be killed.
 * Actually, this happens when one Immortal is beheaded by another. In the third Highlander film, an Immortal is killed by Guillotine, and there's no fancy light show.
 * In the novelisation, Thulsa Doom is revealed to be an ancient Serpent-Man of Yig (using illusion magic to appear as human, as they have the power to do). Which explains the extended lifespan right there.
 * Nope, Immortal.
 * Lots of Conanverse sorcerers can extend their lifespans. It's magic.

Assuming the Serpent-Man of Yig revelation holds true...
...and Thulsa Doom’s true form was the giant snake he turned into in his throne room, not James Earl Jones, then all the snakes that appear in the movie in relation to him are his children. The other giant snake Conan beheads is either another child, a sibling or his mate.

Conan is actually a Terminator.
The movie version, that is. If you make a small boy push a giant mill wheel round in circles for years, it won't build up both sides of his body equally. In fact, it will kill him. But by a strange coincidence just as he collapses a time-travelling cyborg with a damaged memory appears and, for reasons best explained as a system bug, listens to the boys dying words about Crom, revenge, and the Riddle of Steel and imprints on them (in particular the part about steel). This explains why Conan has almost no personality, speaks about twenty lines in the entire film, gives away enormous rubies and feels no pain whatsoever.
 * Why assume the wheel was the only exercise he got for twenty years? He was a slave, they sent him wherever he was needed, the wheel was just the first and last place he worked. Also, the no personality thing is because he was played by Ahnold. The no talking thing was because the producers loved his muscles and hated his accent. And Conan really does do things like give away treasures on a whim and stoically resist pain in the original stories.

Conan is the son of Crom.
This explains his nearly superhuman attributes, his combination of invocation and disrespect towards gods in general (familiarity breeds contempt), his resistance to magic and mind control, his inability to find a home among any mortal culture, and his legendary status. When his mortal shell "died", his soul was taken into Crom's mountain.
 * That actually makes a lot of sense... Sort of like a Hyborian Heracles, eh?
 * Ties in with the Red Sonja post above. Kalidor is Conan after he's ascended to God-hood.

Conan is Marv
...somehow.
 * More reasonably maybe Marv is Conan's direct descendant or his reincarnation.

Thulsa Doom was deliberately setting up Conan to become a mighty hero

 * Burn down his Doomed Hometown, make him a slave, and then encourage his drive, his thirst for vengeance...
 * Why, I'm not sure.
 * It was his magnum opus, he wanted to make someone with the will to depose himself.
 * Taking that statement further; His intention was to include himself in a legend that would be far grander and last far longer than even his unnatural lifespan. The Heroes and villains of legendary tales are remembered equally well, sometimes the villains better.
 * He was bored with power.
 * Alternatively, he made it up at to avoid looking bad.

Conan is a mutant
Like the "Son of Crom" theory (which I also like) it explains things like his being able to survive (and win) battles against EldritchAbominations and to chop off limbs with a single stroke (and in one story through an iron bar) without his blade ever getting dull. He unconciously projects a field around any edged weapon that effectively turns it into a monomolecular blade. Plus of course slightly superhuman strength and more than slightly superhuman endurance.

The Hyborian Age is actually the Age of Man from The Lord of the Rings.
It's mentioned that after the elves sail away to the Undying Lands, Middle Earth will become ruled by humans, eventually leading into our own Earth. The Hyborian Age was also created as a fictional ancient past of our world. Furthermore, some of the geography between Middle-Earth and Hyboria is remarkably similar.
 * If we look at this from a grander scheme, this also means that The Lord Of The Rings is canon to the Marvel Universe. Which is awesome.

Conan is Riddick
...somehow.
 * Probably a descendent. Back in the early 00s there was talk of resurrecting the Conan franchise with Arnie as Conan the King and speculation was Vin Diesel would be playing Conan's son, Conn.