Okay, let me take this point by point:
1. The creators, or titans, or whatever we want to call them, are a group. They are 'only mythical', and present a sort of nameless fear. They don't need names or titles. This is something that could named differently in different places - the demons, the evils, the black winds, etc.
2. The overgod sealed all the titans in the center of the earth (possibly in the netherrealm). There is no way of getting out of that. But there is some way that they can communicate with the surface (I'm thinking volcanos, but trees or corruption sites or anything will do, really). They build a secret following among the cult.
3. I don't understand why we need the other races. Humans are there, and the whole story can be based around humans. Would someone enlighten me as to what other races would add?
4. The cult leader could either be a normal human infused with titanium, or some special type of monster from the corruption zones. It definitely should not mess with any of the groundwork that was already layed down (no half-bloods, no bloodlines, nothing else).
5. Overgod - either male, or female, but not genderless. We have to humanize him/her to show the connection to the people.
I appreciate the creativity, but let's not tear down what we have just finished constructing.