If you go to Dalaran crater in Hillsbrad Foothills and stand around any of the mages there long enough, you'll start hearing them say things like "The magic used to move Dalaran was too strong, we cannot hold it back forever!" or "We must dissipate this power before it is noticed by elf or demon" or "We must contain the arcane power we have unleashed", not exactly quoted but something along these lines.
I think that Old Dalaran is now like another Maelstrom, it's a beacon of energy for any demonic power out there and the Legion will invade through it. After all, in Warcraft 3, when Archimonde destroyed Dalaran, he says in the cinematic "you built your cities over stolen knowledge" or something like that.
And Dalaran was built over a nexus of Ley lines, which are like the magical veins of Azeroth.
REPENT! REPENT! THE END IS NIGH!