Ok as Ottomans, I've rivaled Hungary, Mamluks, and Qara Qoyunlu. Other options I had were Venice, Poland, Lithuania, Austria, and Aragon. I chose those three due to their proximity, I figured I'd wind up in it against them sooner or later, so I wanted those bonuses. Hungary and Mamluks also start off as rivals and enemies, so I figured I'd return the favor.

I can't go after Byzantine until a month of game time has passed, it seems. So I'll wait a until I can do so. I see why I'd want Constantinople! How do I get a CB against Byzantine, or do I just declare without? Is there a way to justify the war?

Throughout history, poverty is the normal condition of man. Advances which permit this norm to be exceeded — here and there, now and then — are the work of an extremely small minority, frequently despised, often condemned, and almost always opposed by all right-thinking people. Whenever this tiny minority is kept from creating, or (as sometimes happens) is driven out of a society, the people then slip back into abject poverty.
This is known as "bad luck.”
-Robert Heinlein