Draw a card. For the duration of your turn, your hero is immune to damage
Open to suggestions, but I think this opens up a lot of choices for Warlock we haven't seen so far. Granted, the new card warlock is getting helps, but this would be a cool addition. Whadd'ya think?
edit: I'm reluctantly willing to make it 2 mana card. If you count in the value of the card draw, that means that the hero invulnerability effectively costs 1 mana, with over 10 potential damage negated.
However, considering it needs to combo with specific cards in order to have an effect, and the extreme power of other combos in this game, it might not be unreasonable.