Valve seems to be working on a new game, titled Citadel. If that rings a bell or two in a hopeful regard to Half-Life, well, you're not wrong at all, because the Citadel is a very prominent element of the popular game series.
The datamined Valve game has recently been spotted via code references in one of the latest DOTA 2 engine updates, with pointers to stealth-related gameplay and AI pathfinding thingamajigs. Could that mean anything? We suppose it could, yes.
According to Tyler McVicker of Valve News Network fame, Citadel includes various references to stealth, AI pathfinding, and a top-down minimap. Furthermore, McVicker claims this is not the huge VR release that Valve has under the wraps, yet to make a reveal.
While this may well related to Half-Life in this or that manner, chances are high that it's not much of anything. It's not like citadels are a particularly rare thing to find in games, and it's a small wonder we haven't had a game with that title yet, to boot. As it stands, the only thing we can say for sure is: only time will tell.