What do you mean by "custom user rights"? Do you mean simply adding/changing the tags/labels that appear in users' profile mastheads or do you mean giving them actual permissions to do certain things on the wiki? The former can be done with one of several dev wiki scripts. The latter can be done by contacting FANDOM as Applemasterexpert said. However, you need to have a really good reason for the change. Wikia will not make changes to the default permissions just because you want it.