My idea:
What if the U.S. and other western nations took a hands off approach to Syria, Afghanistan, Iraq and Iran. We would do this by pulling out all of our troops and military advisers and stopped all bombing missions. We would still support friendly Arab nations like Jordan, etc and Israel too.
Noble idea but as someone said, it's better to fight terrorist as far away from your borders as possible. Imagine ISIS spreading and actually getting some important city or capturing Nuclear Weapons...
This could remove the 'western thorn' in ISIS side. ISIS then would loose recruiting ability, as they could no longer say to young Muslims: 'see the western infidels are occupying Islamic land, that is a sin'. Without 'evil' westerns occupying Muslim land ISIS looses some of its steam.
That would work, if they were sane to start with. Spain, parts of Italy and all of Balkans at different times belonged to Muslims (Moors, Ottomans and so on) so I doubt they would stop. Also their full name is "Islamic State of Iraq and Levant" , this is Levant:
Light green is for country's that are sometimes not included in the term Levant but I doubt ISIS would care about that. Once done with Middle East they would look for N.Africa, then eventually Europe through Spain, Greece or Italy.
As important benefit would be without the U.S. and western forces involvement, the Sunni Muslim lead countries would be forced to deal with ISIS and do their own dirty work. These Sunni lead countries are afraid of losing power to ISIS.
Sunni country's are supporting ISIS, such as S.Arabia and Turkey. Maybe they want to connect all Sunni states? But when it comes to Turkey it's simply to get rid of Kurds.
Now, to keep it short here is some stuff from top of my head. USA needs to stop S.Arabia and whoever has biggest influence needs to do same with Turkey, maybe EU and Russia, that would stop ISIS from getting money and weapons. Then we would need to find final solution about Syrian Civil War, because Syria can't fight and make a counter offensive if it has inner fighting. I would support Assad as
in my opinion he is a strong leader and not in to the religion, as I said before, Syria had no religion restrictions, you could eat a pig, drink alcohol, girls with tattoos etc. This is why I wouldn't support "moderate rebels", they are cutting heads as much as ISIS, they are religious and would probably be end of religious freedom of Syria. So there would need to be a solution that will unite Syria and then air and supply support would be a huge help with Syrian forces on ground. ISIS is very weak when it comes to military and rule mainly over deserts.