I'd like to see them at least start off as hostile and totally intolerant of the Horde, even if they don't remain that way in subsequent patches/ content.
Though it has been a really long time since their disappearance, so I wonder how much they've clung to old hatreds - they might be far less intolerant as a result of distancing themselves with their past and being forced to deal with whatever's been thrown at them. Then again the recent True Horde shenanigans hardly paints the current Horde in a better light than the Horde Turalyon and Alleria knew, and could justify resurgent feelings of hatred or distrust.
Still, I think that regardless of how their view of the Horde may change, they ought to remain Alliance heroes rather than become "world" heroes.
It'll be interesting to see how Alleria reacts to Sylvanas allying with Orcs and Trolls, and of the Sin'dorei.