I think you need an inciting event to kick the Alliance off. Just having them suddenly decide to attack might be possible with some members, but I doubt Anduin would go gung-ho on immediately attacking what is for all intents and purposes a civilian city.
Before the Storm is adequate in that regards, but I feel an even stronger motive is needed. Something has to be the big inciting incident.
I also wholeheartedly believe that the most wasted storyline with potential in BfA is Saurfang's identity crisis about being a warrior for honor, who for a second or third time has been willingly dragged into a war of aggression.
Losing this aspect would also be a major loss to the Horde, who I think needed that storyline to really firmly place them in the good guys category, rather than the only ocassionally good one.
Hard to combine the two though. I do however think it's possible.