Best case scenario? The power struggle between Iran and Saudi Arabia leads to the formation of several tense, semi-stable Sunni and Shia autocracies similar to pre-revolution Syria. There will be regular, short spurts of violence and terrorist, proxy wars, and widespread human rights abuses, but law and order will remain.
Worst case scenario? Multiple failed states (Yemen, Syria, Lybia, Iraq) that are much like Somalia today: horrific places to live, breeding grounds for violent extremism, and a safe haven for terrorist groups. The Islamic State might actually be able to consolidate territory and establish itself as a real, if unrecognized, state. Within its borders, the worst human rights atrocities seen in nearly a century would surely take place. If the Islamic State were to create a stable, functioning state, they could turn their attention to launching terrorist attacks at Western countries, using their ample supply of Western citizens currently fighting as footsoldiers as battle-hardened terror operatives.
Israel would likely continue establishing settlements and all hopes of a two-state solution would evaporate. Its economic and military strength, however, would ensure that it would never face an existential risk.