Simple Solution to Iraq Constitution - Do What America Did
The only way Iraq can have a workable constitution that doesn't allow one religious sect to dominate and abuse all others is to do what America did - create a secular constitution and keep religion out of government.
America was the first secular government in the world and has remained so for over 200 years in spite of repeated attempts by religious groups to grab power and turn America into a theocracy. (What "repeated attempts by religious groups to grab power" you ask? Under president Abraham Lincoln, the religious republicans wanted to rewrite the constitution to say the American government derived its power and authority from god rather than from the people. Fortunately for America, Lincoln, who never joined a church in his life, managed to rebuff this religious power grab. It's an ugly irony that modern religious republicans are calling themselves the "party of Lincoln.")
Why has America managed to retain its secular government in spite of the fact a majority of Americans are religious? Because the different religious groups don't trust each other! (Does this sound similar to the situation in Iraq?) And they should not trust each other. All the flavors of christianity and all the flavors of islam each believe they are the one true religion and each believes they have a duty to take over and "save" everybody else.
All the various religious groups in America have historically realized that if religion is allowed into government, it's only a matter of time before one sect becomes dominant and begins to suppress and eliminate the competition.
This is the main reason America has retained its secular government for over 200 years - each religious group has understood that its own survival might depend on keeping religion out of government.
This will probably be the reason the religious republicans fail in their current attempt to take over our government and turn us into a theocracy. So far, the religious right has managed to fake out all the other religious sects into thinking they are working for a common cause. They have tricked the catholics into joining the republican anti-abortion crusade. They have tricked the Jews into voting republican to maintain American support for Israel. They have tricked the mormons into believing all "christians" should be united on religious values. But make no mistake about it. The group currently in the driver's seat is the evangelical christians. Evangelical christians consider mormons and catholics to be on the fringe of christianity, if christian at all, and they certainly don't consider judaism and islam to be part of their group.
If the evangelical christians succeed in grabbing full power, you can be sure they will start hammering catholics and mormons and Jews and muslims just as much as they're currently hammering atheists. There will be no room in the government for any religion except "the one true religion."
All the religious groups outside of evangelical christianity had better wake up to the fact they are dining with the devil. If they join forces with the evangelicals, pretty soon they will find themselves on the inquisitor's torture rack.
Likewise, in Iraq, if the new constitution is based on islamic law, one religious sect will survive and the other two will be eliminated. Members of the two weaker religious sects in Iraq had better start pushing for a secular constitution or else they will soon find themselves either dead or converted.
When it comes to religion in a theocratic government, only the strong survive, whereas in a secular government, each religion has an opportunity to grow based on its own merits.
For an excellent history of religion and secularism in America see "Freethinkers" by Susan Jacoby.
See also: How To Fix the Mess in Iraq