Many 'HD' re-releases of older games fix them to run easily on modern systems, tart them up a touch, then pop them onto (virtual) shop shelves, but Age of Empires II HD has actually brought the game back into active development. Sixteen years after AoE 2's initial release, AoE 2 HD has received its second new expansion. Age of Empires II HD: The African Kingdoms [official site] just came out, adding four new civilizations with four new campaigns, along with new maps, units, and, y'know, expansion stuff.
The African Kingdoms introduces the North African Berbers, the Malians from West Africa, Ethiopia's old Aksumite Empire, and those dastardly invading Portuguese. Each have their own unique bonuses and units and all that, plus their own campaigns. You'll also find improved AI, new terrains, a rebalancing, new objects to plop down in the scenario editor, and... it's an expansion, you get the drill.
If you have got the drill and wish to make holes with it (play the video game), you'll find AoE 2 HD: TAK on Steam for £5.59, thanks to a 20% launch discount.
Developers Forgotten Empires are also working on a new expansion for the revived Age of Mythology HD, exploring Chinese stories with 'Tale of the Dragon'.