A countdown timer is currently running on the Team Fortress website and it’ll finish up in about 14 hours (01:00 BST on June 18th). The page doesn’t give much away; all it has is the countdown timer and a monitor with a test card on it.
Of course, that hasn’t stopped fans from running wild with their ideas. The Moonbase is the most likely result of this; Valve confirmed in April that it was working on an update that would introduce new gameplay elements.
Alternatively it could be the End of the Line update or a Team Fortress Classic update. As always, we’ll have to wait for Valve to be ready to show what it’s working on. Let’s just hope the timer doesn’t utilise ValveTime.