The Shiites came close to ethnically cleansing the Sunnis from Iraq, and the western countries begged and pleaded with them to not do this. As it turns out, after years of power and wealth in Iraq, the Sunnis were not inclined to share and share alike, and wanted to rule over the Shiites again.
So, I imagine the eventual outcome will be the Shiites forcing the Sunnis out into likely Jordan or Saudi Arabia.
I don’t think it’s as simple as that. One of the challenges in Iraq is that there has been a lot of intermarriage between Sunnis, Shiites, and even Christians over the years — especially among the more secular groups who don’t identify strongly in a religious sense.