Update: This piece has been updated to clarify that AOE 1's civs will remain separate from AOE 2's roster.
Age of Empires 2: Definitive Edition is set to go back in time with its next DLC, but, like, in a different way than usual. Return of Rome is a "completely new type of expansion pack" that will bring AOE 1 into AOE 2 when it releases on May 16, adding all 16 civilisations from the first game and its Rise of Rome expansion pack to the second game (but in their own separate section, so you can't mix AOE 2's factions with AOE 1's).
That means you'll soon be able to play the (take a deep breath, get a glass of water) Assyrians, Babylonians, Carthaginians, Choson, Egyptians, Greeks, Hittites, Macedonians, Minoans, Palmyrans, Persians, Phoenicians, Romans, Shang, Sumerians, and Yamato on the plains, steppes and tundra of AOE 2. And if that's simply not enough human history to choose from, Return of Rome is also adding one brand new civ—the Lac Viet—to its faction list.
It's not just a factional affair, though. The expansion will also introduce three new campaigns to show off a few of its new (old) civilisations. There are campaigns for Sargon of Akkad (the Sumerians), Pyrrhus of Epirus (the Macedonians), and Trajan (the Romans). If I remember correctly, 'pyrrhic victory' is another way of saying 'really good victory,' so that second campaign will probably be chill as hell.
Return of Rome will also bring in the D3 game mode, "inspired by a ruleset made popular in Vietnam". I confess, I'm not familiar with this one, but the AOE wiki tells me that D3 is a mode that cuts down on rush tactics and drags games out a bit: You're only allowed to fight with a single unit until the Bronze Age, and walls and towers are banned.
Aside from all that, the expansion will contain a bunch of annoyingly undetailed "visual improvements," "UI improvements", and enhanced CPU-opponent AI.
If that piques your interest, you can find Age of Empires 2's Return of Rome DLC over on Steam and the Microsoft Store, where it's currently 15% off to lure you into preordering it.