Was BATTLETECH as the only early unlock for the current Humble Monthly not enough for you? Good news, you can now play more right away as they've expanded it.
Just added today alongside the full game of BATTLETECH are two expansions: Flashpoint and the Shadow Hawk Pack. That should keep you going until you decide if you want to pick up Urban Warfare (not included) and the upcoming Heavy Metal expansion. BATTLETECH supports Linux and the expansions do work fine in my own testing.
The other brand new early reveal (and one you can play right now) is Sonic Mania. While it doesn't have Linux support, it does work in Steam Play and it's a good bit of fun. I have some seriously fond memories of playing Sonic when I was much younger on the Sega Mega Drive while I was in hospital but that's a story for another time…
You can pick up the Humble Monthly here if interested.