I think it would be very good to be able to rename an existing sovereign. Right now if I want to pick a sovereign and change its name, I have to go through the process of creating a new sovereign from scratch, which is long when all I want is just changing the name.
So what I did to achieve that was look for the save file, change the names inside the xml, find out that this was useless, and then rename the xml file itself. It's a bit awkward at the moment, even though it works.