Half-Life 3 doesn't look like it's coming any time soon, so why not take Half-Life 2 and transform it into something completely different -- say... an RTS?
That's what modders Vortal Storm decided to do with Lambda Wars, a mod that was recently greenlit on Steam (of all places). The mod is now available for download here.
Vortal Storm has been working on Lambda Wars for six years. Six years. That's an insane amount of work for a mod. Actual assets from Half-Life 2 have been combined with top-down freeware title Alien Swarm to create this RTS. You can play as a commander of the Resistance or attempt to break down the rebellion playing as an overlord of the Combine.
Might as well kill some time waiting for Half-Life 3 to come out. I'm guessing you'll have a lot of time to kill.