So, as we all know, currently next to nobody uses most warlock cards, because of their drawbacks and attempts have been futile to optimize them so they wouldn't be so strong or weak. This idea might be a bit like the Shaman mechanic "Overload" but with a twist.
So, when you play a demon, that would be too strong for its cost, it "TAINTS" one, or some of your mana crystals. In your next turn, you can only use that tainted crystal if you also pay one health per tainted crystals used.
So, for example, you play a Pit Lord at turn 4. It cost 4 mana, and is a 5/6 (I might be wrong here). But instead of hitting you for 5 when you play it, he will tain 5 of your mana crystals. Turn 5 comes and all of your crystals are corrupted. Now you want to play a 2 drop. You pay 2 mana and 2 of your own health to play it.
This means, that you could use these tainted crystals, (unlike Overloaded crystals) but you will be taking one damage per crystal you've used. Of course, if you don't have all your crystals tainted, the game would always expend untainted crystals first.
So practically, that Pit Lord deal 5 damage to you, but it's up to you at what pace do you want to take that 5 damage.
Of course, with a new mechanic, they would need to optimize the strength of demons and their penalties. However I think, it would be a more predictable way to punish you for getting out stronger creatures early.
This could also apply to some spells, like hellfire not hurting you minions, but tainting your crystals instead.
Edit: Once you use up a tainted mana crystal, it gets cleansed by the next turn, so it's not permanent. Forgot about that part, sorry.
I also have an idea for a new demon:
Is a 1/5 for 2 mana, plus taints 2 of your crystals
Can't be targeted with spells and Hero Powers.
Any minion it hits will be silenced
I don't know if it's too strong for the cost, I didn't really think about the numbers here, I just wanted to think of a card that has similar abilities to that of a Felhound in Warcraft 3. Post your thoughts about the idea.
Also, I think Shadow Bolt and Drain Life could be merged. Like with a lot of Druid cards, you have a coice to do one thing or the other. I think, since both costs 3 mana, you could have a coice to either do 4 direct damage or deal 2 damage and heal your hero for 2.
Please, give your feedback on my ideas.