I don't know if there was ever mushcode for Ars Magica. Google shows old pages that referenced Pax Magica, but I believe that may have been a MUD.
As for Crossgen Comics, is there a way to just base an original theme off what the Mystic comic had? High fantasy place, guild drama throughout the realms between mystical guilds competing to control the power/resources/what have you.
Just looking at Mystic, it looks pretty in depth. It looks like Crossgen had a few comic runs going in a blended universe with lots of lore throughout all of them? It also looks like it used the rare wizard type (Sigil Bearer?) as a special character type (Jedi/Gandalf and his kind/etc).
For me, I'd more buy into original theme these days I think when it comes to high fantasy (most themes actually). I may be odd person out with wanting original themes.