The dataminers at steamdb.info have made a potentially exciting discovery for classic RTS fans: the unreleased entity currently known only as "ValveTestApp221380" is probably Age of Empires II.
This came to light when the app was updated just yesterday with a boatload of new achievements, which refer to many of the factions from AoE2 and its expansion. You can read the full list on Reddit . There is no indication of a release date, but the influx of achievements suggests that it might not be too far off.
It's been 14 years since AoE2, but it's still considered the pinnacle of the series by many fans (I'm partial to Age of Mythology, myself, but it's a very close second). You can still see the complete edition on the bargain shelves at quite a few retailers here in the U.S., so it isn't like it's been unavailable for a significant period of time. As with all older games, however, there are inevitable compatibility issues on operating systems a decade or more removed from the original release. A Steam version could address some of these issues.