also internal problems like if your country is always going to war a new group develops and battles your own army for peace
Rebellion. They should also be able to take a PART of your country only, and create an independent state. If a state has been taken over, it's people will try to take it over, unless the attacker supresses the population enough, for example with it's armies and dreaded lords, or make the people like the new chivalrous rulers.
Traits: Your military commanders and governors (or atleast the top of these, and maybe more, like diplomats) should be able to gain traits based on what they do. A ruler that protects it's people, will for example lead to a shiny knight, who inspires people to fight for him, and that also increases morale. You can also pillage your own people, which gives dread, which supresses the population, decrease the growth, but for example, stills fear into the enemy soldiers.
Each country should start wiht a perk or somthing like that maybe like mass produce tanks without iron somthing like that.
Like I said, countries ought to develop specialities when not going to war with everyone they come across.
Maybe random events like acts of sabotage or protests or a coup somthing like that, also population happyness or maybe election percentage,
Makes the game complex, but alot better, and it will require skill and tactics to keep your people either on your side or supressed to "support" you. Best suggestion there is :)
also internal problems like if your country is always going to war a new group develops and battles your own army for peace
This can be taken in with the happiness factor. Being tired of war decreases morale and happiness, but increases experience and stuff like that
Strategy: People shouldn't only use strategy's on military fronts, but also on economy etc... They could also "hire" or elect secondary players to join a certain country, and for example, the first one leads everything a bit in general, a second one gains huge might in the military, the other is like the diplomat etc.... That way, players can make themselves usefull and work together in 1 country.