Croteam is finally starting to give out some basic details on their next game, now called Serious Sam 4: Planet Badass.
Details are obviously light, but we finally have the official name along with this teaser:
When I said basic details, I really did mean it. This is literally all they've shown.
Given their history with Linux support, I've no doubt it will be on Linux and it will likely be using their newer Fusion engine with Vulkan support. It does list "SteamOS + Linux" in the system requirements section, but it's obviously not filled out yet. Checking SteamDB quickly, it does show that Linux is in the "oslist" which was added recently—a reasonably good sign.
There's also now a Steam page ready for it.