Basically, I suspect that ISIS will be used by the CIA and western "coalition forces" to keep supply routes open in eastern Syria.
I imagine that the bulk of US forces will be concentrated in the northern Kurdish regions of Syria in post Assad Iraq. It is unlikely that Turkey will ever accept Kurdish entities in northern Syria (Iraq is bad enough). The original plan for subverting Syria was to snake a Saudi pipeline through Iraq, and then through the Kurdish and Sunni Muslim minority areas of northern Syria, and on to the Mediterranean Coast. However, I do not think Turkey's current dictator will accept a situation where Syrian Kurdish entities will financially gain through this pipeline arrangement, and thus gain a better capacity to threaten Turkey. In light of this assumption, I suspect that Turkey will use the Al-Qaeda (NOT Isis) linked groups in Syria to put genocidal pressures on Syrian Kurds, and on the United States indirectly.
The CIA will need to use its ISIS friends to keep eastern Syria open, as well as the Iraqi province of Anbar, so the Gulf nations can keep land and air corridors open between northern Syria and the Gulf States region. I think Turkey will make it very difficult to resupply US forces, and its allies, from the Turkish / northern Syrian border.
It is possible that Mosul is being used to tie down Iraqi forces, enabling them keep the western regions of Iraq free from Iraqi Army pressure. If Iraq does decide to fully lean toward Iran, we can see a step up in ISIS activity in Iraq, as well as letting the Iraqi Kurds take Mosul. They will probably just quietly tell ISIS to "leave" while the Iraqi Kurds publicly announce that they making an offensive to "help the Iraqi government".
Currently, the Iraqi government has Mosul surrounded, breaking down the last pocket of ISIS resistance in the city center. The Iraq Kurds are off to the northeast. It will be interesting to see if some ISIS west-east offensive comes "from nowhere" against the Iraqi forces. If this does happen, it can happen while Assad is in power or out of power (if that ever does happen).
It will also be interesting to see if there is some "negotiated" withdrawal" of ISIS pockets from western Syria...I wonder where they will end up...? hmmm...
https://en.wikipedia.org/wiki/Battle_of_Mosul_(2016–17)#/media/File:Battle_of_Mosul_(2016–2017).svg
Even if Mosul falls completely under Iraqi *** control, my understanding is that it has large Kurdish minority population, and will be a sensitive ethnic fault line between Iraqi Kurds and the *** dominated Iraqi government, which will result in a constant continued pressure along that front for the Iraqi government. This will just tie down Iraqi military resources that can be used against the CIA's ISIS friends.
The western "coalition forces" may also keep a tenuous hold in southern Syria, just over the Jordanian border, but they will be under enormous pressure from Hezbollah factions and Syrian Army remnants that form more secular focused *** militias. If the US army does try to pull a fast one and take Damascus from Jordan, they will not be able to hold on to it for very long. It will be IED meat grinder warfare all over again, and Trump will be under enormous political pressure to atleast pull back US forces to an "advisory" role. They will likely try to push it off on Arabic forces, mainly from Jordan and the Gulf monarchies. I wish these fellows luck...however, in this case, I imagine that the CIA will use its friends in ISIS to help put pressure on these *** militias.
Brian Ghilliotti