Hypothetically, what positive aspects could come out of the next 50 years of Middle Eastern-US policy if ISIS is successfully able to create and defend territory between Iraq and Syria?
It will be a counterbalance to Shia axis (an effective one, unlike the current Sunni powers that basically are largely ineffectual and mostly amount to asking for help from USA).
It will possibly attract the most radical and hardline Muslims away from other countries, the same way communists tried to move to USSR.
It will provide a sustainable basis for stabilizing geopolitics of Middle East - at least its Iraq portion - by carving it up into separate homogeneous Shia, Sunni, and Kurd pieces.
Plenty of people see this as a beneficial long term outcome, though of course that's hard to assess as a hypothetical.
It will increase anti-Islam sentiment in the West, by showcasing the worst sides of it - emanating from the purest implementation of it, based on Atalntic's expert analysis in "What does ISI really want".
Depending on one's point of view, that may be a benefit or not.
Not likely, but the consequences of #4 might just be the catalyst for Islam to undergo its own Reformation, the way Christianity did in early 17th century, formalizing and establish-ising less hardline and less fundamentalist versions of the religion.
It gives USA and the rest of the West a much better target.
If they have a reason/excuse/need to strike at ISIS, it's a LOT easier to find a meaningful target if your opponent is a stationary "real" state - with real and tangible material assets, infrastructure, and leadership who are comfortable and thus sedentary - as opposed to a militant movement which is far lighter on all 3.
It allows study of different societies as a comparison. If Islamic STATE continues to exist, sociologists can compare and contrast people's GDP/capita among different societies and we will have objective proof whether it is good or not. I predict it will fail miserably. It can be held as an example of a mistake never to be repeated like the holocaust or Venezuela Chavismo.
It can also act as a sink or sponge for sociopaths around the world, drawing them out of productive societies and concentrating them into a localized area.
There no such thing as "successfully implementing" the Islamic State. IS inherently wants to be perpetually at war.
It's in its core philosophy that it can't accept borders with its neighbors.