What about adding a summon 2 Blastmage apprentices, that will act like small Ragnaroses but being 3-3 that deal 3 damage at end of your turn (maybe 4-4 that deal 4 damage) would tahat make it worth playing?
That's probably a bit much. 3-3 or 4-4 are between 3 and 4 Mana worth of stats and an effect like that is worth about 2-3 Mana by itself. So you are immediately summoning an additional 10 to 14 Mana worth of stats onto the field. That might just be too much.
In the first place I would just change the Hero power because 9 Mana for a 3 damage hero power is just overpriced. Maybe if that one was akin to the Ragnaros one or maybe even a split one, like 2 Mana Deal 8 Damage randomly split across all enemy characters.