the Monolith’s obelisk-like deity possibly being, like, a satellite thing sent by aliens that crashed out of orbit a thousand years ago

the real interesting thing about that is that it's basically a leftover from the real early stages of development (before even the Chernobyl setting had been settled on), when it actually was just a 2001: A Space Odyssey monolith, complete with the whole idea that it causes beings arounds it to "evolve" (which was how the mutants were explained away). They changed it later on, but still kept that aspect of the lore as speculation by in-game characters