The Beast and Gruul: Very good both, maybe the beast 9/8, because a turn 6 with this is very hard to remove and also gruul too, but nice to see a good buff in gruul.
Nozdormu: Very very broken, I would want to see something like 8 mana 6/8, just this, the way you made is just insane op.
Onyxia: Same, it should be 8 mana 6/8 or 6/7 With maybe "your dragons have +1 attack".
Captain's Parrot: If you make a pirate deck this is the same of "draw 2 cards". Just buff +1 life in it and it would be better.
Gelbin Mekkatorque: Doesn't need mana cost change and doesn't need buffs in stats. He only needs best inventions, maybe a 6 mana 6/5. poultryizer (don't know the correct names) at the start of your turn transform an enemy minion into a 1/1 chicken. The chicken becomes 0/2, the machine that restores life 4 life to a friendly character, and the machine that buffs things buff a random friendly minion with +1 attack or +1 life at the end of your turn/each turn and with a better body maybe 0/5.
Icehowl: Agree with the cost of 8, but 10/10 is already good. Maybe a 11/11 for something different :DD.
The Skeleton Knight: You just made the worst legendary card (in my opinion) transform into one very insane card, this is just insane man. a "5/5" if you count the stats 7+3 = 10 "same" as 5/5 for its 4 mana cost and an amazing deathrattle. The only thing I want to this card is better stats. 5 mana 6/4 or even 6/5 or 7/4 with same effect, maybe 6 mana 6/5 with deathrattle: "reveal a minion... if your costs more, return this to your hand costing 2 less", maybe even a 5/5.
This is my opinion, all other cards are very balanced and some cards are very creative better than the originals that exist in hearthstone.
The others are very good, I really liked quartermaster and new version of bolvar(it can also become an insane card, but this only time can show us) .
Mindgames: I don't know why you put it as a common, I really prefer it as an epic.
Holy Fire: Too much, 5 damage 6 health is enough, the card is already good.
Shadowform: In my opinion this should be 3 mana *change your hero power to deal 3 damage, if already in shadowform, deal 4 damage to a character* (4 damage not with hero power, 4 damage as a normal spell with target).
Betrayal: 1 mana is very cheap, I think the card should cost the same and have something more, maybe deal 1 damage / 2 damage to a minion and the effect?
Sinister Strike: 3 damage is enough. Gang Up as 2 mana is balanced, 1 mana is a bit op. (Maybe 2 mana put a copy in your hand and 2 in your deck maybe)
One-eyed Cheat: 5 mana sounds too much, I prefer this as a 3/2 or 4/1. In a pirate rogue the effect is very good for too much attack like this. You can also compare to magma rager.
Perdition's Blade: Good, I don't like this card too, maybe 3 mana 3/2 would be balanced.
Headcrack: I used to think this card is the worst spell in the game, but it has a lot of value, I think it should be 3 mana (like the actual) but you can choose any target you want, not only the enemy hero.
Poisoned Blade: This card is difficult to discuss, I thin the best way it's to make it 3 mana 1/4.
Dark Iron Skulker: 3 damage is too much, make it a 4/4 with same effect and it will be much better.
Edwin VanCleef: I prefer the actual way but yours is good too and don't die easy to bgh.
Kidnapper: With the combo this card is the same as "destroy a minion" and you also gain a copy of the minion, I only would want better stats on it, maybe 4 mana 3/3 or 5 mana 4/3, it has a big value but very bad body for its effect/cost.