Both Alleria and Turalyon have been out of circulation a long time now - the Azeroth they're going to be returning to isn't the Azeroth they left at all. I imagine they'll be heading a neutral third-party faction either together or apart, probably associated with the new Class Orders (Turalyon for Paladins and Alleria for Hunters). The Alliance of today isn't Turalyon's Alliance, and the same is true of Alleria and Silvermoon. Like Tirion before him I think Turalyon would see the Legion as a greater threat beyond Alliance/Horde partisan conflict, and Alleria would be ill-disposed to join either group given her history.