Here is a card that will define and dominate all odd-cost decks in this meta. Baku is a legendary that upgrades your hero power in the same way Justicar Trueheart had done before.
Since it's also a beast, it will potentially see play in Hunter, but the one class that proves this card to be really good is Mage. Then, the hero power with 2 damage can be upgraded to 4 damage with the help of Clockwork Automaton. And there you have it!
The last but not least mention is Control Warrior that relies on upgraded hero power, which is now a possibility once again.