They could spice things up a bit by giving each class an alternative hero power. Kinda like "going for a different talent tree" in WoW.
For example, the Mage could have a freeze ability that does no damage but makes the targeted minion deal 2 less damage for 1 turn.
The Warrior could have an alternative ability that is aggressive rather than defensive like shield block is. This aggressive ability could spawn a 2/1 weapon or give any currently equipped weapon +1 damage.
The Paladin could give +1 health to a friendly minion at the expense of -1 health to himself.
The druid could give +1 attack to a minion at the expense of -1 health to the same minion.
The Rogue could put a poison on an enemy minion that does 1 damage per turn for 2 turns. The minion also has +1 attack while poisoned (at least for the first round, maybe both rounds is a bit imbalanced).
The Shaman could spawn totems on the enemies side. These totems have random negative effects that screws up the opponent but they also have 1/1 stats so they can be used against the shaman's minions in a kamikaze fashion. One example of a negative effect could be "Deals 1 damage to a friendly minion on its left side once per turn". I say left because when he spawns it, it will appear to the right of all your minions, but any minion you spawn will end up on the right side of it, making those minions immune from being hit. It sounds weird but it's kinda like having the ability to spawn a 1/1 token minion with charge and immunity (because its magical damage, not physical), at the expense of the enemy being able to attack back with it. One other example of a negative effect it could have, is making spells cost 1 more mana. Or minions costing 1 more mana. So which totem you get is random, just like how the normal 3 totems are random.
The Warlock could heal itself, or a minion, for 3-4 health, at the cost of 1 card (that you get to choose, maybe? Or perhaps it should be random).