Two years later, Age of Empires II HD is still going strong. So strong that, according to producer Ryan Chaply, more people have spent 50+ hours with the game than there are actual humans living in Little Rock, Arkansas (~200k, says Wikipedia). Better give them something new.
The community team behind The Forgotten, an expansion for Age of Empires II HD released in 2013, is once again collaborating with Skybox. Their next official, still-unnamed expansion will add "new civilizations, campaigns, game modes, units, and more." And if you want to stick with the vanilla game, "you can look forward to additional support and features that benefit every owner."
More immediately, the developers pushed out an update yesterday that should improve team battles and game performance for lower-end machines. Also, support for Windows XP is out (RIP).