On this point, in the RFC mods for Civ4, Rhye placed the Ottoman capital in Sogut, which is somewhere over here:
https://maps.google.com/maps?q=sogu...&t=h&hnear=Söğüt/Bilecik+Province,+Turkey&z=6
Alternatively, the modern capital of Turkey is Ankara, somewhere over here:
https://maps.google.com/maps?q=Anka...&t=h&hnear=Ankara/Ankara+Province,+Turkey&z=6
Better still, if you go back far enough the Ottomans came out of Central Asia somewhere. Putting both the Huns and the Ottomans in that area would help fill a big blank in the map and free up crowded territory in Europe.
Though none of those would really be "Istanbul," and ultimately may look a little weird. Just thinking out loud