Might be too late to include this (if at all possible), but could work as a DLC.
Something that could be included in D4 could be something like a military conflict? Obviously it doesn’t make sense to fully implement detailed war mechanics, that would essentially be like creating another game. But you could get away with something that is both rather simple (in comparison to games focusing on military conflicts) yet also adds an interesting dynamic to the game.
There are already situations that involve the military already in game, but I’m talking about something a little more involved. You wouldn’t need to manage any specific troops/movements/battlefield strategies. The main mechanic could be as simple as having a bar that goes from loosing the war to winning the war. The war could be triggered through low foreign relations and/or low democracy markers. It could even be a civil war if violent crime/extremists gets too far out of control.
Increasing military spending could move you closer to a win, but neglecting things at home (such as having a food shortage) would effect the troops and pull you down closer to a loss. Choosing the already existing nuclear weapons policy could increase deterrence. Authorising the use of nuclear weapons while in conflict could help you to win a war but would also decimate your international perception and your standing amongst groups like environmentalists or liberals. If you use nukes against a country that also has nukes you might suffer a lot of damage to your infrastructure requiring more spending on roads etc. Any conflict might increase the need for medical attention and thus medical demand.
You could even make it slightly more complicated and have three bars for land, sea, and air. If the “air conflict” bar goes down too much it might make air travel within your country more dangerous, so road/rail might become more congested as a result. If the “sea conflict” bar goes down too much, there might be a blockade and international trade plummets.
The populance may become unhappy due to the negative effects of the war, or they may rally around their leader and become more patriotic. Triggering a war could risk falling public opinion, but a victory in the war may secure more resources.