I think almost every druid deck would play The Emerald Nightmare in its current state. If I understand correctly, this attack doesn't disappear off of your turn, so even without the other benefits, it effectively says "whenever a minion attacks your hero, deal 4 damage to it."
Aggressive decks would love this card because of the aformentiomed defense against other aggro, and requiring you to attack every turn isn't much of a downside when going face costs you nothing. For those decks, it might as well say "deal 2 damage to the enemy hero every turn."
I haven't made custom cards in a while, but I think if you want to make this less problematic while keeping the flavor, you can have the attack only be on your turn, and require the hero to attack a minion every turn (if possible). Maybe reduce the attack bonus or remove the double damage to minions at that point, since 4 damage can kill a lot of things, especially in the early game.
I think almost every druid deck would play The Emerald Nightmare in its current state. If I understand correctly, this attack doesn't disappear off of your turn, so even without the other benefits, it effectively says "whenever a minion attacks your hero, deal 4 damage to it."
Aggressive decks would love this card because of the aformentiomed defense against other aggro, and requiring you to attack every turn isn't much of a downside when going face costs you nothing. For those decks, it might as well say "deal 2 damage to the enemy hero every turn."
I haven't made custom cards in a while, but I think if you want to make this less problematic while keeping the flavor, you can have the attack only be on your turn, and require the hero to attack a minion every turn (if possible). Maybe reduce the attack bonus or remove the double damage to minions at that point, since 4 damage can kill a lot of things, especially in the early game.