We are talking about international politics, how could morality come into this? No one "deserves" it. First of all, we need to set some standards: *why* should anyone get Constantinople/Istanbul? Well, in my view, our goal should be to try and minimize conflicts, deaths or wars.
So it definitely shouldn't go to Turkey, that pogromed Greeks and Armenians in the 1950s, as well as before that. People were evicted, I don't think we should give it to them.
At the same time, and I say this as a Greek, under no circumstances should it be given to Greece; we would have done the exact same thing, just look at Greek Macedonia and Thrace to find all those Bulgarian or Slavic populations; Greece also had a nasty view towards minorities.
Now, Russia? The same Russia that is in a multi sided civil war, with the likely winners being totalitarian radicals, or, through extensive butterflies, authoritarian military dictators? Let's not.
That leaves us with two options: Free City, or Bulgaria. I have absolutely no idea, what Bulgarian policies on minorities were, other than having heard that they took care of Jews in World War Two, though I definitely haven't checked that with any sources. However, Bulgaria was also on the wrong side of the war, wasn't it? Wouldn't it be politically difficult for a member state of the Central Powers, to actually gain territory after the War, without having fought for it? As I understand it, the Bulgarian Army was on the brink of collapse as it was, it would hardly be able to fight Turkey and Greece and the odd great power for Istanbul/Constantinople.
Do we then make it a Free City? Well, wasn't that what the Treaty of Sevres tried to do, and failed utterly? A Free City would need constant backing from at least one great power that would be willing to involve itself in a war over it. Ideally, in my view, a Free City would be best, but you would need a great power backing it, as well as the people supporting it. That would also be quite difficult.
So here's a radical idea: make it the new seat of the LoN, and force every great power to station troops and a fleet or two there. Put it under the direct jurisdiction of a LoN-created body, maybe not a democratic body, that would ensure life would go on in a multicultural environment, without pogroms, ethnic cleansing or the lot. That would open up huge questions of course, but I can't for the life of me find a better solution.
