to create cities - You can't create cities in this game
or other titles - You can create titles to a certain degree although it is rather limited.
-You can rename government positions(there are 4 of them for every realm - ruler, judge, general, banker) as a ruler
lithe limit is the population right? - No. Population has nothing to do with titles except under one of the systems where you can have different titles according to your population. Not sure which one though. Even then it doesn't really do much. In this game, Baron = Count = Margrave pretty much.
can you rename a realm if you take over one? - Well if you took it over through a rebellion, yes. But pulling off a successful rebellion is pretty tough.
