I'll go with option d) (although I really despise it): Her evil soulfragment will get vanquished and we somehow retrieve her "good" soul which then will carry on being leader of the forsaken, two birds with one stone. She won't get the new arbiter, she isn't free of emotion and therefore unfit to judge souls unbiased.
Would absolutely fit the current quality of WoW storytelling.
Also, after reading the current mmo-champ frontpage: everyone ready for Arthas coming back?