If you work for modders, modders will work for you.
A couple of new features would help advance the modding community:
1) An event editor overhaul to make it more flexible. Currently, the event editor is so bugged that text will quite simply run off the screen. Here's what would improve the event editor:
Fixing obvious bugs (text running off the screen, conditions not working [Controlled by, army in province are examples of conditions not working], the 'decision taken' condition displaying an error message even when valid, et cetera).
Fix the 'Repeatable' glitch: Even when the conditions for the event are not met, the event fires anyway. Properly implemented booleans on events would prevent this.
Adding a search field (also known as a search box) so that modders are able to search for events and organise their events better (especially handy when the number of events exceeds 100).
Adding an event that is applicable to multiple participants with the following fields:
Random Civilisation: Selects a random country once the event fires.
Multiple Civilisations: Actively checks and fires the event (once conditions are met) for all countries in the list.
Any Civilisation: Actively checks and fires the event for any country in the game. (Would be great for flavour events to keep the game fresh).
Every Civilisation: When any country meets the conditions necessary to fire the event, all countries will receive events.
Adding a new effect that would allow for buildings to be built on provinces.
Adding a new effect that would annex land to a civilisation regardless of the province owner/controller.
2) New editors for existing mechanics. This would open up brand new possibilities for modders.
Building Editor: Enables for the creation and editing of buildings, and the various effects that they would give.
Technology Editor: Enables for the creation and editing of technology, and the various effects that they would give.
3) New mechanics/features.
Capability for multiple units. This would ideally come with an editor which could create new units and adjust such things as cost, manpower, manoeuvre, offence, and defence. These units do not need unit models. Simply place an icon to the left of the number to represent which unit they are.
Culture/Religion. Culture or religion would go a lot farther than the current nationality system, allowing for more complex behaviour as well as a better assimilation mechanic. (Right now, you simply throw money at people to make them 'assimilate', cultures would need to be 'accepted' or 'tolerated').
Casus bellis. Also known as 'justifications for war', casus bellis would put an end to the mindless wars that the AI tends to do. These would also come as event effects, and causes for war could be edited by modders via a new editor, or a .json file. Casus bellis would significantly improve the AI, and do away with the current concept of 'AI Aggressivity'.
Loading screens (Not necessary). Custom static loading screens are extremely easy to implement in Java. It requires fetching a raw image from a file and simply rendering it on top of the screen. These would make the game (as well as mods) look far more professional than it already does.
4) Steam Workshop.
At the end of the day, mods need to be distributed somehow. And the best way for mods to reach a wider audience is through Steam Workshop. Combined with previous reforms (which should be relatively easy to do given that the core framework of the game is already implemented), mods of much higher quality than the ones we see right now would begin to appear.
Conclusion: These changes to the game would completely overhaul the game for the better, revolutionising the way mods are made and drastically improving the quality of these mods. In the end, it's the modding community that makes games successful, and that applies to the realm of Grand Strategy as well. Players are looking for features, and only you can deliver on that. Work for modders, so that modders work for you.