Ya the obvious is elemental mage. It seems to fit and will see play. The question is, does the card warrant space in decks? Its similar to Happy Ghoul and even that card doesn't see much play. The saving grace is the elemental tag. The synergies in mage just keep stacking. This feels like it should have been a mage card... playable.
We've seen quite a few times that cost reduced/free minions tend to be at least decent. Arcane Golem, Thing From Below, giants in handlock, and even Moonglade/Firelands Portals which are a similar sort of thing (albeit the minion is random).
Having to play a spell that costs 5 is a bit of a hindrance, and will ensure that decksmiths will have to find the right decks for it - but the power is certainly there. Just imagine - Firelands portal into random 5 drop of around 5/5 stats, plus two other 5/5s (these dudes)? 5 points of damage and 15/15 stats for 7 mana is pretty crazy.
It's going to be hard to justify a card slot for this one, at least for the next few months. A vanilla 4/4 that basically comes out on curve (even if, yes, you can do something extra that same turn) just doesn't have much impact, I feel.