The Shiites in the south of the country, while not pro-U.S., could move into a transitional type state simply by separating them from Iran with some device such ass an international (screw the UN, make it British or coalition occupation) occupation zone around the coastal area of Basra. The zone would be mainly to control trade. Want a crappy Islamic state? Fine, as long as it doesn't harbor terrorists or anti-western Madrassas, the zone remains open and trade and your lifeline will flow. Screw with us and the spigot closes.
The central so-called Sunni Triangle remains the biggest question mark and source of unrest. Two options remain insofar as I can see: