We should be able to request with our allies to do much more things than preparing a war. As well as trade.
For example, insult / improve relation with another country, change ideologies(this one should be added to ultimatum too), scientific research etc.
Also, we should be able to help our allies in development, economy, building, and science.
About vassals, they should listen to whatever their master says. Master should be able to declare war on the vassal if they reject.
We should have some non-historical formable nations too, such as Benelux, European Union, East African Union as well as African Union, Arabian Union etc. But we also need to be able to form historical nations too. Such as you could form UK as England or Scotland, Germany as any German states, Netherland, and many more.
I know a lot of this is moddable and customizable, but wouldn't be great if it was in the base game?