I personally really hope that you make more sets like these because it was awesome to look at those cards and discuss with you about them.
Just as a last thing, I think Seiza is now a bit too good against Control... the draw a card effect is pretty important. What prevents Innervate from being OP is exactly the fact that you need two cards to play... with Seiza, you only need one card and you replenish even more mana later on. I think it will be fine being a 0 mana spell with the text: Refill 2 empty mana crystals. Flow(2) Refill 4 instead. This way this card would still be greater than Innervate later on but it wouldn't be OP... drawing a card would make it really good because you actually get a 4 mana discount without even paying a card... it's basically free. It would be an Innervate that is a worse earlier on and better later on... I think it gets balanced this way.
NOW I really didn't see that you've made Starcraft cards and I REALLY REALLY REALLY love that game... I MUST see them NOW!
I think nerfing Ancient Preserver to 7 mana would make him quite bad... I would prefer if you nerfed just the life by one then, making him a 6/9 for 6 mana.
About Reghar Earthfury, I wouldn't like it to be BGHable... I think you could just buff him to a 6/8 with the same text or make him a 4/7 with the text: Turn any damage done to your hero into heal. It would be like Mal'Ganis but it would cost one less and not have the demon synergy. I like it this way...
About Fel Cleaver... no, it isn't more OP as a 4/1 with that text for 3 mana. You just need to compare it to Deadly Shot: they fit similar roles, but the difference is that this Fel Cleaver would be a more risky and greedy option... you get a 4/1 for the cost of the extra randomness. The big difference between the two is that this Fel Cleaver is insane while you are behind, but dead when ahead... Deadly Shot is just good regardless... so, it's not OP. BUT a 3/2 seems reasonable to me so you could nerf it to a 3/2.
Spiritual Burst seems balanced to me in the way I suggested, Forsaken Berserker would still be quite powerful as a 3/7 and Bloodthirsty Berserker would be fine as a 7/9 I think... the BGH weakness would make the card less OP for the cost.
About Flow, yeah I really think you should choose just one part of it, and at least to me the flow +X seems much more simple and awesome as a mechanic. I would choose it.
About the hero power, I would nerf it to Bleed a minion or your own hero. And I think you should put between parenthesis that it is the last of the End Turn effects to trigger.
Anyways, the pleasure is mine to give feedback... I love theorycrafting so it's being awesome to talk with you. The thing is that I'm not that creative to make cards myself, but I'm analytical enough to critique and suggest and try to balance cards.
PS: What about changing Injured Blademaster to a 3 mana 4/7 with the text: Deal 3 damage to itself and bleed it?
Oh, thanks for the feedback apretiation! Yeah, I was wrong about Reghar Earthfury, there are some great synergies that you can do with him. However, in order for these synergies to occur I think he could be a little cheaper and have greater stats as it's ability doesn't quite do much in terms of the board unless you have the proper cards, which are already a cost and a card to pay for... what about a 6/7 for 7? Or even a 4/7 for 8 with the text: Turn any damage done to your hero into heal. That would fit the lore a bit more, make the card better for the cost and have a slight synergy with Bloodthirsty Berserker.
About Meditation, I really liked the option presented by you but I would make the flow be +2 instead of +3. However, I still prefer my own suggestion because it can be used on turn 2 if you really need a card or later as card draw... I think it fits even more with your own thoughts of making the decision aspect of the card.
About Forsaken Blademaster, I think a 3/6, or maybe a 3/7, for 5 mana would be good. 3 attack because the bleeding counts as +1 attack, so the card would be effectively a 4/6 with the potential of further synergy.
About Bloodthirsty Berserker, nerfing it to a 6/9 seems a fine option but I still prefer something else. I thought I had included this suggestion in the post above but as it seems I didn't. My suggestion was that it was a 5/8(possible nerf to a 5/7) for 5 mana with the text: Enrage: +1 attack and deal 6 damage to your hero. Would work quite similarly to Pit Lord but would gain the advantage of gaining +1 attack or even being silenced because it's no longer a demon.
About Ancient Preserver, I think that 6 attack and 10 health are too much for 6 mana. I would either make him a 6/9 for 7 with the text: Can only attack minions. Whenever this card is bleeded, double its attack. OR a 5/9 for 6 mana with just the text: Can only attack minions. OR a 5/8(possibly nerf to a 5/7) for 6 mana with the text: Can only attack minions. Can't be targeted by spells or hero powers.
About Spiritual Burst, I would neft it to a 4 mana spell with the text: Deal 2 damage to a minion and its adjacent ones. [Flow +2] Deal 3 damage instead. As it is, too OP compared to the other pieces of removal already in the game.
About Fel Cleaver, I'd nerf it to a 4/1 weapon for 3 mana with the text: Deathrattle: Kill a random minion. It would be much less broken this way.
About Wind Walk, I think it's balanced in the way I suggested. But yes, it was double Savage Roar double Conceal previously because the text was(I don't remember the words exactly but the idea was this): Give +2 attack and stealth to all friendly minions until the end of your next turn. That meant that the +2 attack and the stealth would last for this turn and you next(2 turns) and that any minion that attacked in the turn this card was played would immediately re-stealth after the attack. I don't know if that was your original intention but that was what you wrote.
Moving on from card balancing, I have 2 other suggestions:
1) Remove the flow -X, just keeping the flow +X. It would be much simpler, easy to understand and would work similarly to overload. Example: Flow:(2)
2) The hero power as it is is kind of a power creep over Mage's hero power given the potential synergies it gives. My suggestion is to make the bleeding effect be the last of all other turn end effects. This way, the hero power would have kind of an anti-synergy with cards like Ragnaros the Firelord , Sylvanas Windrunner and Kel'Thuzad. Another slight nerf could be change it to: Bleed a minion or your own hero. That would prevent face pings as Mages and Druids often do.
Same thing as Smith: love the concept, apreciate the effort, but I have some concerns about balancing. Another concern is that the flow mechanic might be a bit too hard to understand for newer players, in contrast with everything in the game already.
Meditation: Power Creep over Arcane Intellect. If played on t3, then it is as good as Arcane Intellect. If played later, it is strictly better than Arcane Intellect. So... how is this not OP? Because you have to think, decide? Isn't the point of the game deciding? I'd make the card a 2 mana spell with text: Draw a card. [Flow +3] Draw 2 cards instead.
Wind Walk: At 3 mana it is a insane power creep over Savage Roar. At 4 or even in 5, this card continues to be insane because it gives two turns of stealth and 2 turns of bonus attack... it's like a double Savage Roar double Conceal in a single card! Hated the design... I'd make the card a 3 mana spell with text: Give all friendly minions +1 attack this turn. At the end of this turn, give them stealth until the start of your next turn.
Doppelganger: It's almost a strictly better Faceless Manipulator. For one less mana you manage to copy the minion and give it charge? And possibly even Windfury? The dream is like Sylvanas Windrunner, or Cairne Bloodhoof, or really any minion that you have on the board and you just summon a copy of it with charge! Redesign this card please... too broken!
Spiritual Burst: So it is a 3 mana Consecration that if played on turn 5 becomes a 3 mana Flamestrike? That's the dream of every Control player! But it would break Arena so much that I bet that almost every single time this class was an option it would be chosen just to get this card! It's quite rare for any board removal to get more than 3 minions and what this card does is basically refuse to cover that possibility and instead gain you a 1 mana discount for Consecration(worst case) or a 4 mana discount for a Flamestrike that can even be played a turn earlier(best case)... Aggro would vanish with this card. I'd make it a 3 mana spell with the text: Deal 1 mana to an enemy minion and the adjecent ones. [Flow +2] Deal 2 damage instead.
Forsaken Blademaster: Very cool card design, but the stats are super broken for the cost... it is basically a 5/8 for 5 with the potential of upside given the intense bleeding synergy of the class... I'd make him either be a 5 mana 3/6 with the same text or a 6 mana 3/8 with the same effect.
Bloodthisrty Berserker and Ancient Preserver: While kinda balanced in Constructed, would be kinda broken in Arena because of their low rarity and insane value for the cost. Their battlecries are irrelevant since in Arena it's all about board Control.
Rehgar Earhfury: Nothing wrong about balancing but... this card doesn't seem like a legendary, it seems like a rare card as it's effect is too generic. Moreover... REHGAR IS A SHAMAN LOL! I am not a lore expert but I really think that Rehgar would fit much better in Shaman than here... I think you should get another Warcraft character for this.
Fel Cleaver: Just as a 3 mana 3/3 this card would already be OP as 4/2 weapons, which most often will be kinda worse than 3/3 weapons, cost 4. But now the weapon has the deathrattle: Bleed your hero, which should be seen as an upside, and an ability to kill a random minion, which should be seen as a gigantic upside since you choose when to activate the deathrattle... I'd make it a 3/2 3 mana weapon j with the deathrattle: Bleed yourself and a random minion. If you wanna make a card that kills a random minion, make a 1 mana spell that kills a random minion...
Hanzo Blade: I think it should be either a 6 mana 5/1 with that same effect or a 7 mana 5/2 with that same effect. As it is, it is TOO expensive.