Fortunately, we have appeared to have an influx of Muslims onto this site as of late. Maybe some of them could help in coming up with the Caliphate.
I honestly don't see the Ottoman's keeping much territory in Europe proper, even without the comet destroying Constantinople. At the time that the truce was signed, Russian troops were already at the outskirts of Constantinople.
Not only will the Ottomans be reeling from a significant military defeat, they will be suffering the effects of the global nuclear winter, and a revolt of their Arab provinces. They will have their hands full even without trying to restore order to their european provinces.
That being said, the Ottomen's are likely to be the only safe have for the Balkans muslims as both Russians and the HRE enage in massive amounts of ethnic cleansing cum genocide. As far as the orthodox slavs go, if they are lucky they might be able to be resettled by the russians, but I have a feeling they will be undergoing equally hard times.
There is plenty of time for such a thing to occur. Perhaps it goes something like this. Egypt under Isma'il Pasha (is that the right Pasha?) might immediately seize the canal (i.e. refuse to pay debts) and expand into the Levant, but after his death a civil war over power could happen where you get something where you have four powers in the region:
Egypt that controls Egypt (and the Sinai), Sudan, and perhaps a few other areas in Africa to the south.
A Damascus Caliphate that controls the Levant (sans the Sinai), the Hejaz, and many of the Arabian tribes pay tribute if not allegiance to this Caliph.
A OE that has a death grip on Anatolia, parts of the Balkans, and the land between the rivers (Iraq and Kuwait).
Then there is Persia which hasn't changed much but it has more influence than OTL over the tribes on the east coast of the Arabian Peninsula.
How's that?
I'd like to propose a solution to the "where the hell did the Ottoman Empire go?" conundrum: it didn't go anywhere, it just moved its capital to Damascus. Konstantiniyye is still highly important, but after the third or fourth major siege courtesy of the Russians, the Germans, and whoever else is tooling around there it was decided that the functions of the government should be moved somewhere less exposed.
As for why the title of Caliph is used instead of Sultan, that would be because the increase in religiosity following the Fall made the change of title seem prudent.
I think I'll repost something I'd dropped onto the main Peshawar Lancers Redux thread a little while back.