A federal state, with partially autonomous states, is the only way Iraq can move forward. Lots and lots of countries are set-up this way and the US was originally constituted this way as well. In many places, it works much better than having a very strong central government.
Sharia law is a horrible code of laws on which to base a country's law and order system. The Iraqis know this as well as anyone so it will NOT be the foundation of the constitution. The people who pushed it originally (never really expecting it to make it in) are just looking for a way to save face now while it gets pulled out.
The US with a weak (or perceived as weak) federal government lead to secession. The feds then asserted themselves as King Kong and the thought of secession today is laughable. We should just let the three areas form their 3 nations (or even 2-- Kurdistan and Mesopotamia), set up a base of fly over operations in Kurdistan to keep the Shia and Sunni nations at bay and the Kurds from threatening Turkey's stability and say "mission accomplished."