I wouldn't dust or craft anything at this stage of the game. If Blizzard's schedule hasn't changed we're days from the new expansion announcement and things can change very quick. Hold off for now.
It's more like days + a month. I agree with you that crafting/disenchanting anything at this point is probably a bad idea, but Blizzard is very predictable these days, when it comes to their schedule.
We might get an announcement of an announcement soon, where they talk about what the new year is gonna be like. Although, even with that I don't really expect any news before the balance and ladder patch goes live. And if they stick to their routine as they did for the last 4 expansions, the new expansion will be announced early in March, card reveals will start two weeks later, and two weeks after that we have a final reveal stream on a Monday and the release on the following Thursday, either the 5th or the 12th of April.
I have a golden The Darkness and I'm considering dusting it myself. It served me pretty well at the start of KaC where I was seeing lots of Highlander Priests/Mages/Warlocks in Wild. But if you're a Standard player, I can't see where this will see play after the nerf. Razakus will be tamed (and gone in April's rotation), and Highlander Mages/Warlocks are very few in number.
I'd encourage you to keep it if it was normal, but because it's golden...up to you, do you need the dust?
Should I disenchant it or not? I got most of (at this moment) important legendaries already but I'm thinking of crafting the big spell mage dragon legendary.
Dragoncaller Alanna is "ok", but probably not really worth ditching another legendary for in my opinion. She is really fragile to board clears (of which there are many these days).
I'm not saying The Darkness is much better - but you may want to hold onto your dust either for something more useful, or the next expansion. I have a golden The Runespear myself which I'm holding onto for this reason.
That said, if you are realy keen on trying out Allana, then by all means do it. The most important thing in this game is to have fun and try out stuff. :-) (Big Mage just isn't very competitive at the moment)
I think when you receive a golden, you should keep it and try and make a deck around it, you may even discover a new archetype, i think The Runespear Decks i've come across (like 1 in ever 100 games lol) have actually beat me... XD
The Runespear is actually a decent underrated card. It's just the deck it's used in, Control Shaman, is pretty awful right now. As is the Shaman class in general.