The Cthulu mythos would be a very cool thing to explore, since that really is the source of the Great Old Ones concept, but I don't know if the D&D license can do it. I recall TSR got sued years ago and had to pull the Cthulu stuff out of the Deities & Demigods book. But long before scaly Cthulu swam the great ocean, and aeons before the blasphemous and bubbling form of Yog-Soggoth took shape, there dreamed Mana Yood Sushai ...
"In the mists before THE BEGINNING, Fate and Chance cast lots to
decide whose the Game should be; and he that won strode through
the mists to MANA-YOOD-SUSHAI and said: "Now make gods for Me, for
I have won the cast and the Game is to be Mine." Who it was that
won the cast, and whether it was Fate or whether Chance that went
through the mists before THE BEGINNING to MANA-YOOD-SUSHAI--_none
knoweth." - Lord Dunsany
That kind of fits with the "coin on edge" theme which runs through BG I & BG II.
CThulhu is an official GOO patron in D6D, so he exists in that world.
Cthulhu is vaguely part of D&D, in that he was in the 1st edition Deities & Demigods - removed on a reprint owing to a dispute with Chaosium (publisher of Call of Cthulhu RPG). But he is not part of the Forgotten Realms setting, where BG3 takes place.
Like Tharizdun, he could appear from a different corner of the multiverse. But that is just to big for this story. it BECOMES the story. Who cares about The Absolute when Cthulhu and Tharizdun are in play?!
