There are a couple of "small" things I would really like to see!
Spy glass in the Open World, and also in the Crows nest.
Some more detail in the map and maybe the ability to add certain markers, or maybe a drawing tool to keep track of your sailing directions.
Maybe some animals in the sea and in the air?
Have your crew visible in the Open World, if this would be implemented, maybe having them sing songs like Ubisoft did with Assassins Creed Black flag?
Im not entirely sure if this would work, but maybe have the opportunity to change the colour of your sails? Or maybe have a symbol or something on your sails.