Flatpaks are basically containers, allowing applications to maintain their own dependencies separate from your system. It’s similar to a Windows program shipping with its own precompiled DLLs, helping prevent dependenct conflicts when you go to update something you installed with pacman or yay.
I think Deadlock is pretty up there. That said, it’s closer to Smite than it is a hero shooter. The community-driven character builds mean meta is pretty fluid and it has what I would describe as a very accessible MOBA-centered design. I don’t care for MOBAs much, but to say Valve isn’t innovating here would be disingenuous. I think my only problem with it is that it’s lacking something that makes the gameplay loop feel satisfying, but that may just be my bias against MOBAs talking.