I saw the tulpa and the egregore after reading some Discworld stories and the idea just clicked.
The basic idea is an occult caster wants the power of a god backing him up, but he's too much of a control freak to worship any of the existing ones. Solution: build your own that you can control.
By combining tulpa and egregore techniques he plans to make an entity that is fuelled by the minds of its worshippers. With the right spells , such as imbue with spell-like abilty, it could fake being a god reasonably enough.
With some false priest sorcerors who think getting in at the ground flour of this operation is a good idea, I think you have the basics of an interesting cult.
You should read gunnerkrigg court or american gods, it sort of deals with the same idea.
Why should it fake being a god? Assuming there's some method of ascension in your world, it's totally possible to become the real deal.
I think this individual should seek out a group of worshippers of diverse backgrounds. You'd definitely want to appeal to the bards and skalds because their regaling of the wonder of this being is going to draw in more believers. (i'm assuming this dieties become more powerful the more people believe in them) Bards, Skalds and other artists would also be useful in collecting texts and philosophies around the world and organizing their themes to fit in with the dictates of the leader. Getting them on board first is probably the first step, because the leader could sell them on the idea of creating the worlds greatest work of art (a god, literal perfection, seemingly what artists try to create in the first place.)
Bards are also useful because this endeavor is going to take money. Lots of it. And they can pull in the financiers with the deepest pockets.
Obviously with spells you can do a lot, so bringing in a couple level 10+ casters is going to be important, but you may also want to focus on the alchemists, mechanists and engineers of the world too. One thing that you can do, that gods are seemingly incapable of doing is maintain a physical presence in the real world, so perhaps constructing the perfect vessel for the new god is one way to go about showing what this being can do for you that others cant. There's no reason an egregor or tulpa can't posess this vessel and walk around amongst the faithful.
The occult caster better get used to the idea pretty fast that things are out of his control though, because he's creating a being greater than himself. There are plenty of stories of tulpas deciding they don't like strictly following the orders of their creator. If he gets with the program, he could potentially be a pope of a new world order. If he doesn't, you can read Frankenstein and amp up the stakes by about one thousand percent for what might happen to him.
The end result would be visible if this new god has the capability of bestowing divine power and spells on it's clerics and paladins, otherwise you've just got a very powerful entity.
This cult could also be motivated by some real or imagined slight by one or more gods against mortals. Take, say, a person who believes in the gods but sees all the evil and misfortune in the world as proof that those gods don't really care about the welfare of mortals. That person may curse the gods, and try to usurp their power in order to bring about their own utopic vision. Such idealistic zealots can make very dangerous villains due to their conviction and charisma--they often sound perfectly reasonable until you discover just how extreme their methods are.
(IIRC, there was an ur-priest PrC in 3E that had a similar motivation to steal power from the gods without worshiping them.)
Thanks guys, some great material you have.
To flesh out things a bit:
The caster is a LN guy who has generally been obsessed with very esoteric and eccentric questions and experiments.
His three objectives with this project:
1 To gain the power he needs to alter his part of the world more to his liking.
2 Becoming a god is possible, but he wants to know if he can make something else a god instead of becoming one himself. It hasn't been done before and he dearly wants to know if it's possible.
3 He wants to understand the religious mindset. To his great annoyance he just can't wrap his mind around it and he hopes that by building a religion from the ground up he will finally grasp it.
He figures that no matter what happnes he comes out ahead. Even if he loses control and he gains none of his objectives, he always makes notes, so he can try again.