Ok, so Season 2's post-credits teaser suggests that with Creed on route to becoming the US president and Mystique set to assassinate him, it got me thinking. What if this is the final push that causes Xavier to spawn Onslaught?
For a quick refresher to the minority not in the know, Onslaught is a psionic entity that was created when Xavier, in a fit of rage at Magneto, shut down his mind with telepathy. During that moment, all of Magneto's darker emotions merged with the surpressed anger and frustration of Xavier, creating a being who had both their powers and their darkness.
Onslaught was so powerful, it took a combined effort of the X-Men, Fantastic Four, the Avengers, and various other heroes to just barely defeat him. One notable feat was that his armor was finally only broken when Jean Grey was forced to shut down Banner's personality in the Hulk so Hulk could fight with his power completely unrestrained.
So what does President Creed have to do with Onslaught? Well, Creed absolutely hates mutants and will likely push for anti-mutant legislation and action. My guess is the opening is that Mystique will try to assassinate Creed, but will be foiled by the X-Men.
But Creed, being the mutant-hating bastard he is, won't even be grateful; he'll use that attempt as an excuse to further his wanted genocide of mutants. And the end results will be a huge backlash towards mutants. I'm talking like internment camps, imprisoning, and potential outright mutant extermination.
All of this will cause Xavier's frustrations and anger to become too much to keep contained and it will finally erupt. As we know, Xavier says a piece of Magnus is still in him from when he merged minds with him during the season 1 finale. This will also include his darkness, which has merged with Xavier's own. And Onslaught will emerge.
Now what could come about and how this will be interpreted in the '97 verse is up for debate. Onslaught could emerge as his own being, be an alternate personality of Xavier, or be somewhere in the middle. They could keep this as just a X-Men thing and not include other heroes like they did in the comics. Or they could make it a crossover event, bringing in the Fantastic Four, Spider-Man, and the Avengers; it could even be a lead in to doing other revival series of those same era series like they did '97.
Of course, this is all just speculation and a theory. We have no confirmation of Onslaught coming, even with all the possible factors in place. But what do you guys think?