Thread:Vorknkx/@comment-26005008-20150803211106/@comment-1496755-20150808115348

Commander Keen is (canonically) B.J. Blazkowicz's grandson, so we could say they are part of the same universe. His actual name is Billy Blaze, "Blaze" being a shorter and modern form of Blazkowicz.

As for Doom, it's unclear. Perhaps it could be related to the other two, although it would be hard ot explain how the UAC colony on Mars never discovered the Martians and their cities (seen in Keen 1). We could assume the Martians deliberately avoided contact or something... The protagonist of one of the Doom RPG games is named B.J. Blazkowicz III, by the way.

The Quake universe could be tied to the other three easily, as nothing in it conflicts them directly.

The Commander Keen universe is pretty rich in terms of lore and storyline details - this is mostly due to Tom Hall who is a very creative kind of guy. Like I said, the only thing in it that could contradict Doom is the presence of a civilisation on Mars.