i see the innsmouth raid as the moment where the notion of 'secret history' switches to 'alternate history' (as it's something VERY public) and then extrapolated what changes that would result in re: the government and law enforcement response to cults and their activities.
from there, it was a quick jump to WHAT IF GODZILLA (but not actually godzilla) ACTUALLY HAPPENED IN 1954? and how the governments of the world would collude to cover it up (the end result being that the destruction of tokyo was blamed on an accidental detonation of the third atomic bomb after recovering it from the ocean floor.)
the final piece of the puzzle was, funny enough, 'ghostbusters' and thinking about how the government would explain/cover it up (a terrorist group poisoning the water supply with hardcore psychedelics and detonationg a small weapon of mass destruction in manhattan.)
from there it was all stepping backwards to figure out how other historical events would be effected by these changes.)
the end result is something that echoes 'dark conspiracy' in ways that i didn't expect and i'm hoping to start running it, next month.