Actually they are adding warrior lodges as societies for pagans to the game with holy fury, so that could be used for the cult of mithras.
From what I can tell they seem to be mostly martial orientated.
Yes I know, that's what I'm proposing. The Cult of Mithras is a mystery cult though so technically the membership should be secret to non-members. For most of its initiates it's an impious act to join, since they're worshipping a foreign god in addition to their native ones. It will probably be more than just redoing localisation.