My guess is that he will have a role to play in the Forsaken story going forward, mostly by virtue of being someone who looks more undead than, say, Calia.
I could totally see a situation where Calia and Derek rule the Forsaken together, with Calia being the civil leader and Derek being the military leader.
Either that or he might be related to a plot point i have been considering for a while now, where Daelin is ressurected as well. Probably to serve as a raid boss in the final raid, where Derek, Jaina and Tandred help us. (Tandred because he is captain of Anduis ship, the two others because it would be fitting)