1. The ability to make custom buildings for mods and stuff.
2. Aircraft. Aircraft would be like the ground army but can only be trained in provinces with an airbase. To attack with aircraft you could go into into a special 'Aircraft' map view. Aircraft can move 2 or 3 provinces at a time and can move through your territory, nations which you have military access to's territory and enemy territory. When over enemy territory there will be two buttons which will be kind of like the 'plunder' button for ground troops. One will commence a bombing raid which destroys population and economy and the other, close air support, kills troops. A random percentage of aircraft get 'shot down' after each turn the bombing or close air support happens.
3. Capitulation. When a country loses a percentage of its provinces, every turn after that there should be a random chance of the country capitulating and surrendering.
4. More types of resources.
5. Better AI.
6. Religions doing something (In the game files there are religion icons and a json file so I think this might come soon)
7. Leaders dying.
8. Elections for new leaders (maybe)
that is all my idea so far
edit: and also the icon for democracy should be changed. I don't think a crown would be suitable for it