Shameless plug for Kerbal Space Program, by Squad.
What do KSP and Marathon have in common? More than you might think. Sure, there's that vague "its got to do with space" commonality. But wait, there's more.
Both games (and KSP is a work in progress, but pretty amazing so far) have helpless fellows walking around or riding around. "Accidents" can happen in both games (gives new meaning to the phrase 'friendly fire'). Marathon has some Bobs in green suits, while KSP has at least one Bob and they are all green faced.
KSP credits give a shout out to Bungie Aerospace, among others.
KSP is a NOT an FPS or any other shooter. Its a flight sim in which you first build your rocket or plane from parts.
But what about shotguns? Well, here's a bit of a stretch:
http://www.rockpapershotgun.com/2013/05/24/not-rocket-science-in-kerbal-space-program/