Forbidden Flame seems too slow to be played in Tempo Mage. Anything above 2 mana just feels too clunky for a deck so focused on efficiency, and at 1 or 2 mana is probably too weak for a deck that usually has to choose between Arcane Blast and Mirror Image already. As a 0-mana spell synergy-activator... I dunno. Mana Wyrm, Flamewaker, and Antonidas are just 5 cards, and the effect doesn't seem strong enough for Mana Wyrm or Flamewaker activations alone.
If it sees competitive play, it'll probably be in some sort of control Mage, where the flexibility of being able to go 1-for-1 with almost any minion will outweigh the mana inefficiency, and you'll be more likely to be willing to hold it to combo with Antonidas. With all the Deathrattles from Naxx out of standard, this might be good enough, but it remains to see how many sticky minions we'll have in Old Gods.