The Khemric Titan is too big for a standard Fantasy game, much bigger than standard Dragons, and it was restricted to Monstrous Arcanum for good...
Arkhan isn't a proper Tomb King, he's a traitor aligned with Nagash. He should be ported over to Vampire Counts as a Wight King or Necromancer...
Nowhere is the faction innately restricted to that description in lore. Indeed if it was then the Magmadroth wouldn't have been created as a unit...