Good idea; all you need is for Turkey, Saudi Arabia, Jordan, and Iran to agree to a joint incursion into Syria.

At that point, why not go whole-hog and "assist" the Iraqi government in retaking the country? While the ground forces are there, might as well have a conventional war over who gets to be hegemon in Mesopotamia.

As for the geographic particulars of your proposed DMZ, I'm confused. Besides being concerned more with current areas of control as opposed to clear topological/cartographic divisions and landmarks, it tacitly assumes that IS will never be pushed back in the context of the overall conflict.