A problem with meeting is meeting with an empowered negotiating team with a defined set of aims and the flexibility to modify those aims.
We have a very tolerant multi-racial, sexually equal society.
They want sharia law, masculine led society with no alcohol and single religion.
Now negotiate.
No silver bullet, I have as few workable ideas as the next person.
PS, HS you posted while I was writing. ISIL has neither the organisation, franchise, or will. They have not articulated any terms or conditions.