Once a Knave

Question: Do you believe that some Gods may once have been Titan Avatars? For example, Ra being a Titan Avatar of Akhetaten until Isis tricked him into giving away his unlimited power, or perhaps Sun Wukong being an Avatar of Chaos before Guanyin's Crown gave him the Shen's virtues?

While I think this is possible within the setting (and, in fact, through the actions of the PCs some Titan Avatars have reverted to godhood, while some gods have opted to abandon ship and go be Titans), neither of your examples works for me, and I'll tell you why.

Ra is definitely the kind of cosmically huge and badass figure you could set up as a Titan if you wanted to, though I see why he remains playable in Scion (he's also very involved with his pantheon despite his mad power skillz, and he had one of the strongest cults among humans of any gods in the entire Egyptian pantheon), but Isis' shenanigans, while surely irritating, wouldn't have anything to do with his status as a god or a Titan. You can't trick someone out of being a Titan; Titans are cosmic creatures with a direct hotline into their Titanrealm and Virtues that don't match those of the gods, and upsetting or confusing one isn't going to cause him or her to spontaneously develop normal Virtues and no longer be an expression of their Titanrealm. Severing a Titan's link to the Titanrealm is a big deal and a very involved process and it involves changing the very fundamental nature of the Titan; it's not something they can do by accident, or as a result of being confused.

Furthermore, the myth of Isis and Ra itself illustrates that he isn't a Titan himself at that point; I doubt that Wadjet is a sufficiently powerful figure to take out a Titan Avatar with poison all on her own, and the fact that Ra needs Isis to heal him lest he die suggests that he doesn't have the powers of the Titanrealm at his disposal. Then, too, there's the fact that Ra actively gives his powers to Isis for her to distribute; if they were the powers of the Titanrealm, they're not transferable that way unless Isis decided to become a Titan herself, and while she can be unpleasant to people she doesn't like, I don't think anyone would suggest that she must be a Sun-Titan because of her tomfoolery with Ra.

No, this story basically says to me that Ra is definitely not a Titan when it is happening, and since he makes more sense as a playable god, I'm comfortable assuming he was never a Titan in the first place.

Your other example, Sun Wukong, comes from a different direction: he is clearly originally Titanspawn, which the Scion books confirm and which is pretty evident in his origin tales, but he's not a Titan Avatar. Monkey is a barrel of laughs and does many outrageous things, but he's not a major representative of the primordial and has no powers that suggest he's nearly that big a deal; he's a small minion of some Titan (probably Hundun) that manages to make his way to the top and eventually be accepted into a pantheon once they have acknowledged his power. He could have become a Titan Avatar at the end of that road, but elected, thanks to intervention from the Shen and Devas, to become a god instead, thus preserving his headache-inducing behavior to annoy them for centuries to come.

However! Don't think I'm all negatives and rainclouds here, because despite the examples, I don't think that's a bad idea necessarily. Titan Avatars certainly could decide to abandon the Titanrealm and become part of the pantheons of the gods; doing so would require a very strong shift in their perspective and a very good reason, and it would be an event that had a profound impact on both the pantheon they went to and the Titanrealm they left, but it could happen (and, in our games, has). But this is something that's so big and so gamechanging that it's something we think should happen in play, not offscreen in your backstory; if the world is going to change so drastically, that's something that the PCs should be doing, because they're the stars of the show, after all. Scion hinges on the idea that Scions are the new young blood of the old pantheons and world, bringing with them new ways to use their divinity and new ideals and ways to solve problems; if they want to try to "save" a Titan Avatar, then by all means, that's the kind of thing that badass Scions do! Why steal their thunder by doing it offscreen, making it a part of the setting that isn't based on the status quo they've come to shake up? Why make them no longer the ones who cause those kinds of awesome shakeups?

This doesn't mean you absolutely can't do it, of course. There could definitely be an excellent plot involving a god who was once a Titan Avatar, just as you can have excellent plots involving gods who are on the verge of becoming Titans, Titans who used to be gods, or any other mixture under the sun. Just make sure that you have a good reason for that god to have formerly been part of a Titanrealm; if it's in the service of the story and makes sense, then go for it! But if it's just something you're doing that doesn't have a really strong purpose or that would be much more awesome as part of the story the PCs tell than as window-dressing, save it for a time when its dramatic impact will be greater and proceed with the setting as normal.