Some people seem to think that when Lei Shen Absorbed Ra-den's Power he unknowingly absorbed the Remains of Norgannon (as when he contained himself in rad-den). The Reason for this is when Wrathion Consumed Lei-shen's Heart he had gained Knowledge that even Lei-shen did not seem to know. In this senario remember Wrathion says they have forgotten so the titans could still not be dead but in a state of amnesia as well.
Some claim Wrathion Himself maybe Unknowingly the reincarnation of Norgannon to us as well. The Reason behind this could be he purged azeroth of the corrupted Black dragon flight. When he pursued the Heart of Lei-shen and the power/knowledge it had could be an indicator that if he is Norgannon he may be tying to piece together his missing memories and power.