Firstly I'd say Blizzard is sometimes vague with their expansion names. I mean first hearing Cataclysm was the new expansion name we didn't know what it was. Some people guessed, and got pretty close to the truth (just as people are guessing now and may well be correct.) but it wasn't obvious what it was at first. Same thing here. If this is the new expansion all will become clear once we know what it is.
Secondly there's no reason why Blizzard can't give an expansion a purely metaphorical title and I'd say that the Storm fits with the BL quite well. It is a storm coming to Azeroth, and the pre-expansion patch for BC was called the calm before the storm. So even if it was indirectly and via an old idiom, the Legion has already been referred to as the Storm. The Twisting Nether does have storm connotations, mainly Netherstorm and the Nether is filled with Arcane storms. Or as I've suggested The Storm could be a quite literal storm that's about to rip Azeroth a part if it's not stopped.