Dead But Dreaming

Question: Alright, I really, really want to somehow get the Cthulhu Mythos into Scion but don't actually want to have Big O' Greenie, Nyarlathotep or any of the others in the game by name. So, can you think of any actual mythology Gods or Titans that would possibly fit into those roles personality-wise?

This is a weird question, because I'm not really sure what you mean by "personality-wise". The Great Old Ones don't really have a lot in the way of personality - they're kind of giant terror-inspiring primordial creatures with motivations outside the scope of humanity. Since that's also something you can say about Titans, that's probably close to your answer; Titans are the parallel case to the Great Old Ones in Scion, as ancient, uncaring powers who can destroy humanity just by standing too close to it and have no concern whatsoever for their piddling mortal lives.

Several of the Great Old Ones are likely based on figures from ancient mythology anyway, so you're probably doing pretty well if you just align them with those beings and run with it. One of the neat things about the Cthulhu Mythos is that it was designed by not just one but many human beings, each of whom added their own stories and fears to it, so if you're so inclined you could run it in Scion as being a conglomeration of various fiction stories in which mortal writers collect their own frightening experiences with the divine, interpreting them as fantastic horror and bringing encounters with many different deities and monsters into the same universe.

I'm not going to do a ton of in-depth research into this one for you because this is definitely your bag and not ours, but off the top of my head, Coatlicue, Poseidon, Scathach, Dagon, Summanus and Crom Cruach are outright named as Old Ones or Outer Gods, Nyarlathotep has a clearly Egyptian name in spite of not being directly linkable to any specific god, Shub-Niggurath has been compared to Astarte and Cybele in various stories, Istasha is probably based on the Egyptian Bastet, Mormo is depicted similarly to a Gorgon and shares a name with one of the handmaidens of Hecate, Swarog is clearly based on the Slavic Svarog and said to have been worshiped by the Slavs and Norse, Ithaqua is frequently called "the Wendigo" and easily relatable to Algonquin mythology that way, Sebek is most likely an only minorly changed version of the Egyptian Sobek, Nug and Yeb are probably stand-ins for the Egyptian Nut and Geb, Shaklatal is stated to have been worshiped in Egypt as Amun, Yig and his son Voltiyig are definitely related to the myths of Mixcoatl and Quetzalcoatl and are also implied to perhaps have inspired various other snake-gods around the world, and Cthulhu's daughter Cthylla is most likely based on the Greek Scylla. The opposition to the Great Old Ones, the Elder gods, have a few recognizable names among their ranks, too, including Bast and Nodens, a form of the Irish Nuada.

I'm sure there are more, since even one person inventing a mythology can't help but draw from ancient myth, let along a whole giant sprawl of different authors, but I'll leave it to you to figure out. I'll leave you with this idea: if the Great Old Ones and the Elder Gods are in fact merely normal Titans and gods from Scion's setting under other names, might the stories of the cosmic struggle between them not be mortals, all unknowing, chronicling the awesome struggles of the divine without even realizing it?

Incidentally, while we are big haters of the Atlantean pantheon in Scion and do not want to recommend anyone use them, they were pretty obviously based on the Atlanteans of the Cthulhu Mythos, whose worship of the Great Old Ones and subsequent terrible downfall paralleled the invented Atlantean gods and civilization becoming corrupted and destroyed by the Titans.