Egregore is a concept representing a "thoughtform" or "collective group mind", an autonomous psychic entity made up of, and influencing, the thoughts of a group of people. I have a guardain that started out as a Thoughtform (Egregore).
And the answer to your question is a technique known as Assumption of the Godform. It is among the most impressive, as well as challenging techniques in esotericism. A careful study of a chosen pantheon in depth is often required, along with the willingness to spend a great deal of time in extended meditation and devotion each of the pantheon's deities.
Umm... maybe i'm wrong but i thought God-forms were the most powerful beings. The next thing under God. Like Odin, Hecate, Freya, Zeus, Athena, Set, all those guys? There the most powerful type of entity. More powerful then demons, angles, planetary intelligences, everything. As far as i know there's nothing beyond egregore's that a magician can create on their own.
you can create a god form. it's tedious.. ask Luzifer. I believe he was the first to post about Servitors. I have it somewhere I would just have to find it... and i'm not about to do that (too lazy..)
A god form is rather powerful. It will be able to do things you may not want it to... No longer will you be able to tell it to do things...Ask it yes, but tell it no. Personaly I would rather not loose that control over something I created.
I don't think it follows a strict evolution, like a Pokemon or something. Rather a sigil is merely your intent disguised as a nonsense pattern, like a mantra (Chaos rather than Oriental) is your intent disguised as a nonsense phrase or sound.
A servitor and an egregore are essentially the same, as noted above an egregore is more of a group-mind whereas the servitor is created by an individual.
To create a new God-form you would need to have a pretty sizable cult following it. But therein lies a danger, to many minds feeding into it each with their own slightly different understanding will dilute the original intent and reduce it to a simple portfolio of associations carrying only memetic power.