A couple of weeks ago, Assassin's Creed Wiki user AlifMorrisonudin noticed something very interesting in The Division 2: A poster in the game that he thought could be an Assassin's Creed Easter egg. This eventually led to excited conversations on Reddit and at least one YouTube video explaining why it might be significant.
The poster has a Viking-looking fellow, wearing what could be an Assassin's style cloak, holding a spear and gazing stoically out over what I imagine is the sea, or maybe a winter-swept field or something appropriately Nordic. The word "Valhalla" is plastered across the top, in case there's any doubt about where this guy comes from.
But what really sells it as an Assassin's Creed tease is the orb he holds in his hand, which bears a distinct similarity to the Apples of Eden, technology created by the humanity-preceding race known as the Isu that are basically deus ex machina gizmos that do whatever they need to, whenever it's needed. If you want to get really deep into the lore, a Viking connection to the Apples of Eden was established all the way back in Assassin's Creed 2 through the Nordic goddess Idun.
Taken altogether, it's pretty convincing, especially since Ubisoft has done this sort of thing previously. But what really seals the deal, at least as much as a deal can be sealed when it's still purely rumor, is this Kotaku report saying that its own anonymous sources said a few months ago that the upcoming game will in fact be about Vikings. Kotaku has a pretty good record when it comes to this sort of thing, and the fact that it's willing to go to print with the rumor lends it considerably more credence.
No other details are known (or rumored) at this point, and probably won't be for quite a while. Ubisoft said last year that the next Assassin's Creed game won't be out until 2020.