Hand of Gul'dan
The Mana cost can be paid by Health. Each Mana equals 2 Health. For example, you can pay 6 health to use this card.
"What must we give in return?" "Everything."
Why it would be good for Hearthstone
1. I want to bring a traditional warlock skill Lifetap into Hearthstone and I think this mechanic means that player can choose whether to use health to cast this spell in a dangerous situation or use mana to cast this spell as normal. It provides players with more choices so that I think it will totally change the meta now and attract more hardcore card player to play this successful CCG.
2. I want to bring this mechanic into Hearthstone during the future version about Warlords of Draenor or Legion because this card is from the most powerful and famous warlock Gul'dan. I think it will attract more WOW player if putting some WOW elements into card design.
3. This card can interact with the warlock in the Hearthstone very well. It can help the Hooked Reaver. And it is very cooool during player use it by using health if it has a special visual effect.