I dont really see a possibility that the "hypercube" would be anything else then Venice, due to the PCGamer "most serene" "bombshell" comment.
As to how exactly is it "hypercube" i would have to say, that its probably not realy that game breaking and just a lot of promo hype, although maybe it could be something like Venice not being able to produce units and only able to buy them ("Mercenaries") or the coastal settling mentioned before.
The whole idea of "Horde" civ (moving cities, barbarians etc.) COULD be very cool, and, maybe, if they thought of a way to implement it, they would like to test it before Civ VI in one Civ V... but, like i said before, I dont find it probable, becouse I`m certain that its about Venice.
And one last thing... i find it interesting, that (from what i gather) there is a lot of hype about Venice, but the last civ is pretty much a complete mystery, and we only assume its a NA tribe, couse of Pueblo replacement theory and the Tomahawk theory, both being... well... rather weak theories. That would kinda tell me that either the last civ is a "fan favorite" and people will easily accept it and be happy with it, and it does not require a hype, OR it's actualy ridiculous enough, that they just dont want to say anything about it until the last moment, when it would be covered by the Venice hype... OR we only get 8 civ's. Pueblo got scrathed and was not replaced. They just failed to tell us that.