I have a few suggestions
A)Change of leader through event (I know a lot of people have said this)
B)It would be very good if we could change the names of the provinces for every scenario.Like for example,in a WW2 scenario we could change the name of Kaliningrad to Konigsberg and this change happens in all scenarios too.What I'm saying is an section in scenario editor just like the cores and population editor.
C)Leaders being assigned only to their ideology and not to the country