I'm with the people who say Lilian Voss is worthy of taking up leadership of the Forsaken if Sylvanas were to step down... but not all on her own. She comes off as more of a lonewolf type, and taking on such a heavy burden on her own would require a lot of character development
I think that they are setting Thrall up for Warchief or at the very least leader of the orcs but I suppose we'll have to wait and see. Chris Metzen's big ol' question at Blizzcon has my thinking the first one though.