Re: earlier discussions around Turalyon being ‘tricked’ into being an antagonist, I think that is definitely not the way to go. No one likes their faction to be a pawn for some higher master (see the Horde and Sylvanas for the most obvious example).
Instead, keep it simple:
Turalyon is responsible for the protection of Alliance citizens, and he wants to see the old Alliance restored to its former glory. He is given the means to do so (Yrel and the Lightbound fleeing the collapsing AU timeline) and the support to do so (from Yrel, Genn, Maeiv, the dwarves, the Nobles, whoever else). So he decides he’s going on crusade, intent on recapturing all the Alliance’s lost territories and ensuring the Horde is in no position to destroy another Stormwind, Gilneas, Theramore or Teldrassil ever again.
This is a basic motivation that players can get behind. The Alliance should want to see its lost kingdoms restored and should not trust the Horde to govern itself, and in contrast, the Horde should not want their fates decided and world shaped by an uncompromising belligerent Alliance.
Perhaps the ultimate outcome can be some form of trickery, but the initial motivation should be something players can support.