I think it would be kinda cool to be able to somehow win a hero for your deck.
there would be one hero for each element and you may only have up to two in your deck of different elements. You might be able to pick one once you reach score maybe 1k, then 3k, then 8k and so on.
Since you may only have 2, they are a bit stronger than normal cards. Also, maybe they could somehow level up multiple times, but not my the same upgrade system other cards use.
Maybe they have a 1% chance of leveling up after you use them in a game.
An example for an un-upgraded death hero would be,
Name: Necromancer
Attack/Life: 3/4
Ability: Raise dead (every time a creature dies Necromancer summons a zombie (3/1) on your side of the field.
Cost: 10 death quantums
As they level up their points and skills do too
Necromancer level 2
Attack/Life: 4/5
Ability: Raise Dead (every time a creature dies Necromancer summons a Lich (3/3 ability Sacrifice (eat a skeleton and gain +2/+2) on your side of the field.
Cost: 10 death quantums