Mechabellum has sucked away more of my time than I would care to admit right now, and you too can see what all the fuss is about during the Steam Free Weekend. Disclosure: my key was originally sent by TriplePoint PR.
A strategy game that doesn't take a lot of time to get through, since it's focus is the online PvP battles which generally only take a few minutes at a time. It's like a very cut-down version of Supreme Commander, with lots of explosions and varied unit types but for those who don't have hours and hours to spare constantly. It scratches an itch for me, and it really will suck you into "oh go on then, just one more game" as you want to try out a different strategy on another player.
It's not a traditional strategy game though. It's a round-based auto-battler. You pick your starting units and commander, and then each round you pick new units, upgrade, move things around and more. There's a lot of strategy to it, and it can take quite a while to get good at it. I just love the destruction of all the different unit types going at it. Wonderful stuff.
On Linux with Proton 8, it's pretty much flawless. Works great with good performance and looks awesome.
It will be free to try between December 7th, 6PM UTC - December 11th. They will also be running a discount on the game from December 9th - December 15th. One thing to note, is that eventually it will go free to play though with people who purchase getting some exclusive content.
For all the hours I have in it, I'm really happy with this one. Check it out on Steam.