Ok, going to get a jump-start on people engineering the dragon decks, given BRM gets released in a few days. We don't have all the cards, but we have enough to start reviewing some things in preparation. Keep in mind everything written is limited to the cards we currently have and the cards that have been released.
First, here are the cards that have the mechanic "holding a dragon": (Basically, the cards that probably form the synergy of having a dragon deck in the first place, without being dragons themselves!)
Blackwing Technician, 2/4 but holding a dragon->3/5 (3 mana)
Blackwing Corruptor, 5/4 but holding a dragon-> deals 3 damage (5 mana)
Rend Blackhand, 8/4, but with a dragon... Kill a legendary! (7 mana)
Now, let's talk about the dragons.
The old ones
Faerie Dragon, 2 mana 3/2 (can't be targeted)
Twilight Drake, 4 mana 4/1*
Azure Drake, 5 mana 4/4 (draw card, spell+1)
The legendaries:
-Alexstrazza 9 mana 8/8
-Nozdormu 9 mana 8/8
-Onyxia 9 mana 8/8
-Malygos 9 mana 4/12
-Ysera 9 mana 4/12
-Deathwing 9 mana 12/12
The new ones
Note the dragon egg itself is NOT a dragon, so does not apply for "hold a dragon" mechanic
Dragonkin Sorcerer 3/5 4 mana
Hungry Dragon 5/6 4 mana
Drakonid Crusher 6/6 6 mana
Volcanic Drake 6/4 6 mana
Nefarian 8/8 9 mana
Alright. So, how many dragons do we NEED in a deck to make a deck a "dragon" deck? As in, to reliably proc the hold effect. Here's my results... (I may have gone about this wrong, but I don't think so)
Basically, my hunch is to reliably proc a blackwing technician, for example (3 mana play, with a hand size of 4-5 cards) we need 8 dragons+.
Of the holding a dragon synergies...
-The Blackwing Technician seems like a mandated card. As a 3/5 3 mana, this should reign supreme (especially since 3 drops tend to be weak) and if we can consider actually hitting it 80%+ of the time, should give all dragon decks an early game boost.
-The Blackwing Corruptor also is mandated. It's a 5 drop that "does" something, so it helps early game removal (like against the Shredder, as that was pointed out) and given you can get this effect in the 80% range or so (say as a turn 5 play with 5 cards in hand, with 10! dragons, this is reliable at almost a 90% clip) it's a must have
-I'm not as sold on Rend. The big positive is I think in the late game you can consider his mechanic a near-lock, the problem is he basically is pricey removal. Like I could understand opting for Rend over a Siphon Soul, but I'd still rather have a BGH
So, if we really need 8+ dragons, (ideally, 10) which make the cut?
-Faerie Dragon, given we can run 2 copies, seems like a reasonable early game selection. It trades equally against Mech Warper, Knife Juggler, and most other standard early game plays. It's a totally reasonable card that has the added benefit of being better vs Mages (frostbolt) or Rogues (backstab) than other "normal" constructed cards
-Unless you have other card draw mechanics (like Warlock, for example, which will be a unique Dragon Shell class) I don't see Twilights earning a slot. They are great in Handlock where they are typically a high health minion on 4; not so much in ~any other class (aside from the random Duplicate/Echo/AI mage)
-Azure Drakes are great as always. They are probably going to be the only true card draw for a Dragon Deck, as the decks will have higher mana costs so likely will just try to play on curve. 2 seems pretty clear-cut
-The Dragonkin Sorcerer will likely get tried in a Dragon Priest shell, but most of the time (since it's too niche to make a spare part Dragon deck, for example) it'll be a 3/5 for 4 mana. This is slightly underpowered, even bearing the Dragon name. I'd cut it.
-Hungry Dragon, on the other hand, seems to earn a spot based on it being incredibly beefy. And the 4 slot is clear if you're not using Dragonkin or Twilights. Im going to have 2.
-Drakonid Crusher is pretty meh. I could be really wrong about this one. On the other hand, Volcanic Drake appears to be a great tempo swing type card, plus it curves naturally as a 6 drop worst case scenario, giving a Dragon deck a natural rhythm.
So, that leaves 2 Faeries, 2 Hungry Dragons, 2 Azures, and 2 Volcanic. That's 8. I want 8 as a minimum, but ideally 10 dragons. So, as far as the legendaries...
The most obvious/useful ones are Ysera and Alexstrazza. Nozdormu is gimmicky and not good, Onyxia is slow and likely similar (but less fun!) to Nefarian, Malygos needs a deck built around her, and Deathwing is Deathwing. (He, funny enough, could earn a spot in the deck as a total hail mary)
So, my initial shell is going to be the 5 "hold dragon" cards, the 8 dragons listed, and those 2 slow legendaries as late game. The decks need a little more early game, removal, maybe tech innovations, and perhaps the standard Loatheb/Slyvanas/Dr Boom killer 5, 6, and 7 curve.