What about changing innervate to:
Text: Refill two spent mana crystals.
That would make innervate extend the utility of a mana pool without allowing the use of cards that shouldn't be playable for another 2 - 4 turns. It would also bring the card more in line with the functionality of it's counterpart in WOW. Innervate in WOW is not about INCREASING the size of a mana pool, it's about restoring used mana. (Actually the class that has the ability to increase mana pool size in WOW is the mage - via arcane intellect.) That would still make it a good card for increasing tempo in the early game, while allowing late game combos that are otherwise not possible. For example a druid playing an innervate would still be able to play something like - Power of the Wild - Innervate - Mark of the Wild - giving them a 5/4 taunt on turn 2, which is still a very strong play. It would prevent some of the more stupidly powerful early game plays like though, like turn two - Coin - Innervate - Innervate - Ancient of War.