It isn't a fantasy.
https://en.wikipedia.org/wiki/Islamic_Military_Counter_Terrorism_Coalition
https://imctc.org/English
I'm inclined to think that Islamic countries allying together to fight terrorism in their midst is a good thing. I'm a bit skeptical about how effective it will be, since some of these countries are known as promoters of Islamic fundamentalism (Wahabism in Saudi Arabia's case) or as state-sponsored safe-havens for terrorist radicalism (Pakistan for instance). So it looks to me like they are all saying the right things, but behaving as they see fit. (Most countries do that, they talk a 'globalist' line while tending to act in what they perceive is their own best interest. China's currently the world's champion at that game.)
So I don't think that simply adding up their military strength tells us very much, unless a significant fraction of them are prepared to act in concert. A number of them did join the coalition against ISIS (as did the Europeans) but as always the US seems to have carried most of the load (apart from Syria, Iraq and the Kurds, who were directly threatened).
Other than fighting ISIS, I don't really see the Muslim world coming together to engage in any big military adventures. The Saudis did gather a coalition (separate from the anti-terrorist coalition) to join them in fighting the Houthis in Yemen, but that just fell apart in recent days when a UAE-backed faction favoring bringing back an independent South Yemen centered on Aden fell upon the Saudi-backed faction based in Aden that favored a unified Sunni ruled Yemen, forcing the latter to flee to Riyadh. So right now, Saudi Arabia and the United Arab Emirates seem to be at odds. The UAE seems willing to allow the Shi'ite (and Iranian-allied) Houthis to control what once was North Yemen in order to end the Yemen civil war (or at least the UAE's role in it), while the Saudis violently oppose any armed Iranian surrogates on their southern border.
My point is that forming the Arabs (or the Muslim world more broadly) into a tight military alliance seems to be like herding cats.
Is there reason for military concern in this area? Obviously. It's perhaps the least stable part of the world, apart from sub-Saharan Africa where capabilities are far less. (They can't even control their own territories in many cases, let alone project power across borders.)
Nuclear weapons in Pakistan and soon in Iran are major areas of concern. Next to North Korea perhaps, I think that Pakistan is the biggest nuclear threat on the planet. Pakistan has nukes, is halfway to being a failed state as it is, extremely unstable and infested by crazy militants. It's easy for me to imagine its central government falling apart and the territory turning into a collection of cantons ruled by nuclear-armed Islamist warlords. (I expect that threat keeps Indian military planners up at night.)