The most dangerous thing for the west to do would be to treat ISIS like a terrorist group, possibly even AQ. That would be a big mistake - not only because doctrinally they are more advanced than AQ, but also because their current status is more akin to that of the Taliban (maybe The war was just a rehearsal for A N Other war?). So to analyse them on the basis of AQ would be a misreading of the situation.
For an ISIS perspective, the most dangerous thing they could do now is what nobody expects, they simply stop. They say to the world we are here, these are our borders and we are simply going to consolidate and develop as a state - there won't be any further expansion. Now we are no longer dealing with a terrorist organisation or an insurgency, but a proto-state with views that clash with most of the civilized world, but if they go no further than their stated borders and become an inward looking medieval state that just gets on with living in caves, it does rather leave the west with a quandary as to how best to handle them. Added to all the oil they have (OPEC membership request??).
For my money, watch other airfields in Central Syria go next as a springboard to the central western regions of Syria.