It's hard to improve upon it
Events: One of the good things with the DLC but mods could do it so you're basically already subscribed
Flagships: You could argue improving this but arguably it's such a minor mechanic that the only thing I can see being modded is more options for new countries and being able to build more
Expulsion of Minorities: Ha, the mechanic and AI is so broken, unfixable
Pirates: Game wasn't made for pirates, it'd have to be a major overhaul and just for pirates is meh
Was there anything else that was added lol?