Over on Reddit someone posted that there is a "United Empires" victory achievement. Does this mean we have some sort of co-op victory scenario upcoming?

Yes it is.  This works for allies.

werewolf_nr wrote:

Over on Reddit someone posted that there is a "United Empires" victory achievement. Does this mean we have some sort of co-op victory scenario upcoming?

It's not a "scenario" in the sense Warcraft 3 custom map sense of the word - what "shared victory" means is that you don't have to turn on your allies in the last turns of the game. Instead you can win together  

From our documentation:

• A new victory type called “Allied Victory” will be added - it can be enabled and disabled in the game options screen.
• To win through “Allied Victory” you need to be the direct ally of an empire which wins through any of the other available victory types (you are not a direct ally of yourself, so you do not win an Allied Victory from winning while being allied with another empire).
• It is possible to uncheck all the victory types except for “Allied Victory” – this will simply make it impossible to win the game (as “Allied Victory” relies on another victory type).
• “Allied Victory” cannot be combined with “Elimination”: if the only to empire left in the game are allied an elimination victory is not triggered.

Thanks for the update. I see it dropped today as well.