Iron Fist of Disappointment
Iron fist, while not as genuinely awesome as most marvel characters, is even more disappointing in this game. I suggest a very simple change, that won't make the character ridiculous, but will allow those who would pick him as their favourite, some sort or way to utilize his underwhelming presence.
Let him charge his chi fist in his heavy attack.
Maybe, for each second charged it adds 20% attack only to the immediate attack when you let go and he goes for the heavy hit.
If he is hit while charging, he loses the charge. If he releases and hits nothing, he loses the charge.
If he hits a blocking opponent when charged, it ignores the block completely.