Adding demigods to the game, at this point, requires replacing one of the current demigods. Stardock and GPG are awaiting a sign from the community that it is both willing to spend money to enhance Demigods and to have a attitude that would encourage GPG & Stardock to support further development. The best way for this to happen is for people to create useful mods. Given the tremendous number of ideas alread posted for new demigods, it is doubtful that any new ideas will contribute much. However, making your own demigod would be possible, you'll just need to work at it. The largest problem is that demigod uses 3D models in the granny file format, which is not available for freeware or has an easy conversion method.
The best way to start working on your own demigod is to start modding. Read the threads, all of them. Download mods, see how they do things, take notes, etc. It is a large amount of work, but that's what you would have to do.