Hmmm... now that you mention it, I agree. A 3rd shard should be a creature.
Green shard = regenerate life permanent
Blue shard = gain life + maxhp spell
Red shard = ??
gain life+maxhp when this creature dies?
Heal all your creatures each turn?
Instead of doing damage he heals you for an amount equal to his power?
Gain 20 life when he comes into play, but lose 20 life if he leaves play? (It's a mtg effect, yes, but still an idea.)
Or maybe:
Shard of Power 4
N/N
N = Your life divided by 20 rounded up.
This would make him a 5/5 for 4(Too powerful?) normally, but when used with stoneskin/shard of divinity he would have the capacity to be as large as a 26/26 creature.