I really disagree with effects that effectively say "in X turns, I have you on complete lockdown".
Now while I realize that these hero cards are currently like Mirari's Wake was in M:tG, in that they're rather difficult to cast, but if you do, you effectively win, I disagree entirely with the concept of "oh look, I play X. I win."
The reason is that they're either
A) reliably playable and thus broken beyond belief
or
B) so difficult to put into play that you're strictly better off not attempting to play around them and go for a more straightforward victory.
However, if you're really dead set on the idea of these "hero" cards, then I suggest you do something ala the new Planeswalker cards in M:tG. Basically, here's the concept:
Planeswalkers are literally miniature players. They cost some moderate amount of mana (between 3 and 7 I believe) and come into play with some life and 3 abilities. One ability is their basic one which boosts their life by one. Another ability is one that depletes their life by some variable amount that the player controls. A third ability is their ultimate ability, which reduces their life by a massive amount, but has a game-changing effect.
IMO, heroes should work like this:
They come into play weak. At the end of every turn, they get more life. They have one passive ability, and one active ability.
For heroes here, I say that they should have some moderate stats, and their one active ability should be the signature of the element.
Here's some off the top of my head:
Fire: 5 Fire. 9/5.
8 fire quantum: Your opponent's shield is automatically destroyed. (Errata: this does not target the shield. This empties your opponent's shield slot, period. So it doesn't matter if he has a bone wall with 1000 counters on it. It's gone.)
Chaos: 5 Entropy. 6/6.
10 ent quant: improve all of your critters (fallen Druid's improve).
Aether: 6 Aether. 6/1, untargetable.
8 aether quant: Cast Twin Universe.
Wind: 8 Wind. 12/12
6 wind: All your critters control dive this turn.
Water: 4 Water. 2/5
8 water: some % chance per critter to freeze opponent's critters for 2 turns
Gravity: 8 Gravity, 12/8, momentum
8 Gravity: All your critters permanently gain momentum.
Life: 6 Life, 4/6.
8 Life: Gain 4 life for each critter you have.
Light: 8 Light, 6/10
8 Light: Target critter gets fully healed, is cured of any status infliction, and gets +4/+4 permanently.
Death: 6 Death, 1/5
10 Death: All critters you control gain venom until end of turn.
Darkness: 6 Dark, 6/8
6 Dark: Steal target permanent.
Earth: 6 Earth, 2/10
6 Earth: Pop 3 pillars/towers in a cluster.
Time: 6 Time, 5/5
6 time: target player draws six cards.