I read the reddit speculation thread actually, does make decent points, at the very least the hints wouldn't be Chaos. The main hint is the opening quote of the trailer, which is a quote that apparently is used to define Necrons arrival. In the older codex, it was used for the Tyranids but was later changed to Necron or something, so there's a slim possibility it's Nids, but it's unlikely as it'd be kind of a shitty reveal to have Nids that were in the previous game. The point about the DoW2 recent update adding a Necron Overlord to the Last Stand mode, even though there had been no update for like what, 3 or 4 years, would also tend to suggest Necrons would be a central part of the new expansion.