It SHOULD be Constantinople again, because it’s on the Greek side, and thus, should belong to Greece. Turkey got it by conquest. But it’s a pipe-dream that they’ll ever give it back.
Nationalities need not change. The Turks should allow the Greeks who were driven off in 1955 to reclaim their properties and to be compensated. Also, the church of Hagia Sofia shouldbe returned to the church - which is after all a native local church - and the Turks would make a fortune off the tourist traffic.
The problem is Turks really are afraid someone may come along and kick them out - it is their hidden unspoken fear.