ISIS arose, in large part, because the American-backed, largely Shia government in Iraq was marginalizing Iraq's Sunnis. The same is also true of the Shia-led government in Syria, which has inflicted a brutal war on much of its Sunni population.
Until there is genuine political accommodation between the Shia and Sunnis in Iraq and, down the road, in the even more complicated case of Syria, a son of ISIS will surely arise because Sunni militant groups like ISIS will continue to claim that they alone can really stand up to the Shia-led governments in Baghdad and Damascus.
Also, Iran and Saudi Arabia and its Gulf state allies are fighting well-funded proxy wars in both Iraq and Syria, where Iran-backed militias hold enormous sway. Unless there is some cold peace between the Iranians and the Saudis, this proxy war will continue to fuel anti-Shia militant groups such as ISIS.
The collapse of Arab governance across much of the Middle East also fuels groups like ISIS, because these militant groups thrive in failing and failed states such as Iraq, Libya, Syria and Yemen.
Beyond these macro problems, there are other challenges that will follow the fall of Mosul.