Frost Giant is definitely a possible replacement for mountain giant, because you don't always have a mountain ready to go on turn 4, and mid to late game he'll be cheap no matter how many cards are in your hand or what life you're at. Against really aggressive aggro decks, he'll have the exact same problem as mountain giant, you can't tap that much so he'll stay expensive, same as mountain. There's also the option of cutting boom and something else and going with 6 giants.
And then there's the possibility of just cramming him into existing decks. Might be a viable replacement/supplement for moltens in echo mage for example.