Ironically enough, most of the (non pet) abilities for your Ritualist is actually kind of what THM's skillset in version 1.0's original launch was like - their skills were based around darkness-based spells and using debuffs and drain attacks. It was only when Jobs were added did THM's skill set get completely altered and it gained a whole heap of abilities that were originally on CNJ, loosing the debuffs and drain attacks as well.

The only problem I can see with this class is lore-wise necromancy is a strictly forbidden art (even the THMs of Ul'dah regard such magic as an abomination), so I highly doubt a guild for such a Discipline of Magic could ever be set up in any of the city states. Of course I'm sure Ferne and the rest of the lore team could find some way of having it fit into the existing setting. but I don't see it happening.