Soundwave on 24 April 2013
I don't think Iwata needs to be replaced per se. He seems adept at handling the Japanese market, at least the handheld side anyway.
Nintendo is a company missing its second half though, they need a new person to be in charge of NOA/NOE who makes decisions with consultation from Iwata, but has a lot more autonomy. Nintendo also needs to take into consideration the entire global market when making hardware, not just the needs of Japan (read: Wii U).