No above-ground capital would be safe for the Islamic State. To protect its control center from bombardment, the caliphate would need to bury itself in tunnels, like termites (or al-Qaeda). But even a buried bastille would need to be some 60 meters down to be safe from bunker-busting munitions like the GBU-57A/B Massive Ordnance Penetrator.
Should the Islamic State manage to create a political state with a capital, how closely could it model its governance upon the historic caliphates it claims to emulate? In all preceding caliphates, power was demonstrated, in part, by the number of women the caliph controlled. Hundreds of women were impounded in the palace from which government decisions emanated. Most of the women were not for sexual pleasure, but simply to demonstrate dominance.
At the moment, the Islamic State’s systematic killing of men and taking of women performs as a predatory horde rewarding its warriors more than as an organization developing the governance of a true caliphate. A core question is whether the new caliph will be able to maintain and control the women he acquires as well as his predecessors did. And who will handle the daily governance for the new caliphate to maintain cohesion in the state?