And I think as a chock value, the destruction of Dalaran is a pretty good thing. During Legion, the city was allowed to fly almost above the Broken Shore, where a gunship almost got destroyed by a single Gul'dan spell. While it provided the player with a great scenery of the main threat of the expansion (the Tomb, until it was changed to be Argus), it wasn't really logical to see the city fly like that in the sky.
Now, there's a sweet irony behind Xal destroying Dalaran because the priest player made her see all the plans the good guys had to save the world in case of a situation like that. Also, she probably got a lot of informations from Magni and Khadgar. It was a pretty logical choice for her to destroy the city and there's a lot of symbolic behind that.
The only thing that's bothering me, but again it's here for a gameplay design is the same stuff we had to deal with in WoD, which is the nerubian launching an assault on the city when they haven't cleared the place they're in. Here, Xal's decision to attack Dalaran will probably be pressed by the fact she hadn't planned to be spotted so quickly where in WoD, it didn't made sense for the Iron Horde to launch a global invasion of Azeroth when they still hadn't conquered Draenor.
The only thing I hope is for Xal to use the same spell Archimonde used back in Warcraft 3 to destroy the city. As for Khadgar, I hope he won't die stupidly - if he dies. Even if he was sometime annoying with his jokes, the guy has been a key character for 2 expansions, and an important one for 2 others.