Jason is Decade.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.
Jason's father is alive and back in AtlantisAdmittedly 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 PoseidonJason'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.
The Oracle is Jason's motherBecause pretty much every line out of her mouth could be subtitled "I'm your mum". Seriously.
Jason is of the ArgonautsHonestly, 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.
Atlantis will sink in the series finaleIt's practically a Foregone Conclusion
Hercules will "complete" his twelve labors over the course of the seriesIn his own, Fake Ultimate Hero way
Hercules and Pythagoras are not Heterosexual Life-PartnersPythagoras 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.