If one of these happen :
1) Turalyon and Alleria follow the path of Tirion/Rhonin, and become neutral to the Horde, for the greater good.
2) Turalyon is still Alliance but Alleria finds out about what happened to her kingdom and the Blood Elves, and decides to join Lor'themar.
3) We find out that one of them is dead, or dies in the process of the expansion.
4) They don't show up at all, for another expansion.
If you want my personal opinion on this, i'd like both of them to show up and join the Alliance. Damn, they even have their own statues in Stormwind, to me they are two important heroes of the Alliance.
But i have this feeling...that Alleria will choose to protect her kingdom, Quel'Thalas.
I guess it'll be a tough decision for her : To stay with her husband and her sister Vereesa , or fight for Quel'Thalas...and join her other sister, Sylvanas.