Hearthpwn

This are the legendary's I would like to have, I think they're fundamental for a couple of deck types I can't play right now. 

Baron Geddon ; Al'Akir the Windlord; Malygos (classic set, I own almost every other important cards) 

Vol'jin;Toshley (GvsG) 

Rhonin; Chillmaw

So, right now I think my top priorities are Al'Akir and Vol'jin, but I own this following crap and I'd like to have your opinion guys on what to dust 

Malorne; Aviana; Trade Prince Gallywix; (never been able to fit him in a decent competitive deck) Wilfred Fizzlebang ;Captain Greenskin; Mekgineer Thermaplugg

and some epics: 

double Mulch, Dark Wispers, Gladiator's Longbow, Cogmaster's Wrench, Ancestor's Call, Twisting Nether, Hobgoblin

I appreciate your comments guys! And, I know this cards are or will be fun (or they may never) but I believe GOOD legendaries are even funnier 

Also keep in mind that some of this cards have been through 2 expansions already and still can't fit a good competitive deck 

Legendaries: 

Baron Geddon is only used in warrior decks, and not too often anymore, honestly.

Al'Akir is a static is pretty much every shaman deck so I'd hold on to that. 

Malygos is a "fun card". It sees a lot of play in combo decks, generally mage or shaman. 

Vol'jin is a great priest card. I'd hold on to that. 

Toshley is generally just a decent replacement if you don't have a better legendary. Not really used competitively. 

Rhonin is a 50/50. Some mages use it, some don't. Your call. I have one myself and I'm just holding on to it just in case there's a comeback. 

Chillmaw is just for anti-control in dragon decks. Nothing else. 

Malorne, Aviana, Gallywix, Fizzlebang, Greenskin, Thermaplugg are all pretty garbage. Disenchant at will, unless you'd like to have a little fun with them (such as Greenskin in pirate decks, etc.)

Epics:

Mulch is horrible, not used competitively. Same for Dark Wispers, although the card can be fun.

Gladiators Longbow is just too high of a cost for what it does, playing Dr. Boom is a much better turn 7 play.

Cogmaster's Wrench is fantastic in Mech Rogue, but nothing else.

Ancestor's Call is a fun card for combo shaman decks. Ancestor's Call --> Malygos --> 2 spells and dead lol.

Twisting Nether is not used competitively, although I saw an appearance of it in Dreadsteed Warlock.

Hobgoblin is a great card if you want to have a deck full of 1-attack minions. Not used competitively, but super fun if you can pull it off. 

I hope this helped! :)

If you want good competitive cards like Al'Akir the Windlord and Vol'jin, then go for it. There's no reason to keep cards you never plan on using.

In my time playing iv'e disenchanted my fair share of decent epics and legendaries to get the cards I wanted (I still regret crafting The Mistcaller a little). 

Here's a little more inspiration

Too bad, but maybe he will see play some time....he's not the best but certainly not Malorne...Thanks! 

Just to echo what everyone else is saying, I would disenchant pretty much all those.  And I'd probably prioritize crafting Gheddon, Alakir and Vol'jin.  Though honestly all of these are quite replaceable.  So it's up to you.

You can easily just remake the dust anyways if and when said cards get good. I'd keep the gimmicks (Ancestral Call, Nether, Hobgoblin) just because they can fit in some gimmick/for fun deck later. They have the best shots of fitting into a new archetype and you don't want to have to spend dust making epics if you can avoid it 

Dark whispers has been a pretty decent card for me, in token Druid or sometimes my ramp druid just to buff a taunt (Technically gives Questing Adventurer +6/+6). The standard combo with it is whispers, soul of the forest or whispers, power of the wild, innervate soul of the forest.

Keep fizzlebang and maybe mulch but other than that you can dust the rest. I wouldn't craft toshley though.

dust them all

Quote from ameret18 jump

Also keep in mind that some of this cards have been through 2 expansions already and still can't fit a good competitive deck 

The only thing I can say to you is:

Aguante River, loco! Y el Burrito Ortega también! :P

But, talking about the topic, the problem is that most of the legendaries you listed actually have potential for future expansion, and we don't know still if Aviana isn't going to find a deck that can use her to its full potential in a competitive level soon.

Also, you wouldn't want to disenchant Ancestor's Call if you are planning to get Malygos, that card is essential for a Malygos Shaman deck (not tier 1 yet, but tgt is still young so I wouldn't be surprised if this decks really improves).

For all the cards you listed, I think that at least you can disenchant Cogmaster's Wrench and Mekgineer Thermaplugg.

Que grande el millo! 

Quote from ameret18 jump

Que grande el millo! 

Mañana se lo rompemos a los bosteros :)

I would keep Fizzlebang and Ancestral's Call. Fizzlebang may be a thing in handlock and as others mentioned, Ancestral's Call can come in handy with Malygos which is one of your desired cards.

If you don't plan playing ramp druid, DE Aviana but personally I would keep it. You can use it with Malygos and Faceless for an OTK deck. If you keep Aviana, I also recommend to keep one Mulch if you have multiple.

If you just want those 7 legendaries, then you should be able to get the dust before the next expansion comes. The question is how urgent those cards are for you?  Disenchanting is super ineffective in terms of dust value and since you never know which card becomes viable in the future it might be better just to wait until you gained the dust. If you are really going to de something I would recommend to keep Captain Greenskin and Hobgoblin. There will be more paste cards in the future and pirate decks might become viable. Hobgoblin is a really good card and it can always happen that it will be included in a deck again.