Suddenly ending up in another world, fighting monsters with a mysterious sudden skill, and looking to find out about his origin? Sounds like Decade to me. Of course this means that he will end up destroying the world before saving it.
- He can't transform because his cards are possessed by Circe. That's how she turned Hercules into a pig, she Final Form Rided him.
Jason's father is alive and back in Atlantis
Admittedly the first episode makes this a bit obvious to the TV savvy, but let's get this WMG out of the way so we can concentrate on the who, what, where and why regarding his parentage.
Jason is the son of Poseidon
Jason's father gave him the symbol of Poseidon, he was a diver, Jason is 'more than a man', things are scared of him, he can suddenly turning into a good fighter sometimes. I can't be the only person pondering this.
- It would fit. Theseus was a son of Poseidon, and it could play in to the legend of the city's sinking.
- Didn't Theseus slay the Minotaur, too...
- And the Minotaur said that he betrayed Jason's father, and was then cursed by Poseidon. If Poseidon isn't Jason's father, they're clearly connected somehow.
The Oracle is Jason's mother
Because pretty much every line out of her mouth could be subtitled "I'm your mum". Seriously.
- Jossed in the season 1 finale. His mother is Pasiphae!
Jason is of the Argonauts
Honestly, the writers would've changed his name if it was unintentional, one of them suddenly becoming aware of it. Also, how many Jasons were there in Ancient Greece? Eventually, this'll be how he ends up if for no reason other than all of Greek history and mythology have ended up in an unsunk Atlantis
As of the Season 2 premier, this seems very likely; in his Oracle-induced vision, we get a brief shot of a ship with ARGO written on it.
Atlantis will sink in the series finale
It's practically a Foregone Conclusion
Hercules will "complete" his twelve labors over the course of the series
In his own, Fake Ultimate Hero
Pythagoras has more than once spoken about his "love" for Hercules, which is taken as just a deep friendship, but then in "Hunger Pangs" Hercules mentions he has never once seen Pythagoras with a woman. Not only that, Pythagoras takes quite long stare at Jason when he turns up naked in the same episode. Pythagoras is suffering from unrequited love for Hercules and a case of Incompatible Orientation
Jason is the true heir of Atlantis (and his birth name is Theseus)
It's been mentioned a few times that Minos took power and that Jason would be as good as dead if it was known who really is.
Despite what Minos said, Jason and Ariadne will marry.
The first season finale implied that Jason has a legal claim on the throne of Atlantis (possibly more legitimate than Minos' claim). Rather than precipitate a civil war in Atlantis between Jason and Minos when this fact comes to light, it will be decided that Jason and Ariadne should marry and rule jointly. That way their children would be the undisputed heirs to the throne of Atlantis.
Jason is in a coma.
Everything after the submarine crash isn't real. The flooding of Atlantis will happen either as Jason dies in hospital, or as he wakes up.
Jason marries Medea, not Ariadne.
As mentioned above, it seems likely that Jason is the one from the Argonauts legend since we saw a bit of the ship in the vision the Oracle gave him. The start of season 2 also introduces Medea, Pasiphae's witch-niece, who is a Princess of Colchis, as per the original legend (which also has her as Circe's niece, but hey, if she and Pasiphae have a third sibling, that's not outside the realm of possibility - Heptarian had to come from somewhere, right?) and in the original legend, Medea helps Jason find the Golden Fleece in exchange for his promise to marry her; with Ariadne's decision to marry someone of Royal blood to protect Atlantis, this leaves Jason free to get over her and fall in love with someone else - seems a bit of a coincidence to introduce Medea now, right? Also, there is a heavy implication that Atlantis will have a downer ending, like Merlin, and the tragic marriage of Medea and Jason from the original legends could play into that, with some twists, naturally.
The clincher, for me, was what the BBC America website character profiles said of Medea: "In a world ruled by politics and power plays, she is a lost soul, seeking a connection. As her aunt’s protégé, Medea’s magical gifts are encouraged and her battle skills hardened – but she needs more. It is this desire that will eventually force her to choose where her loyalties really lie."
Pythagoras is asexual (and aromantic).
His lack of interest in women and general lack of understanding when it comes to romance suggests that he may not experience romantic or sexual attraction at all. The very strong friendships he has with Hercules and Jason are the sort of friendships (sometimes called queer-platonic partnerships due to them being beyond regular friendship) that asexual people can form in lieu of romantic partnerships. He's also unafraid to outright say that he loves Jason and Hercules, suggesting that to him love is defined as platonic love.