The difficulty is mostly based on the lack of AI willingness to WC. I made a mod for myself that, among other things, increased difficulty by upgrading AI.
Mostly:
- I gave AI free AE from the start (biggest cheat, but way more potent than +50% manpower or forcelimit)
- I gave them free claim on at least one province of every country in range.
- Two events that changed the ruler's traits depending on the internal situation
- Changed defines so that AI wants more provinces.
The effects were: 1490 Ottoman-MIng war; France forming Rome in ~1700; Ottomans with all of India in ~1650 and so on. Playing with a minor nation near a major one, even on normal difficulty, was impossible.
AI is not playing like a player. It can, but it won't, since it creates an unhistorical world where, in 1600, there are only empires remaining.
The best way to increase difficulty is to increase AI aggressiveness and desire for conquest.
While internal affairs are important, I doubt Paradox will make them challenging. Losing to rebels means losing some of your land. That's quite a hit, and players don't like losing.
Ideally, I would like feudal/governors system, where you need to create pseudo vassals for everything far enough from your capital. Something like EU4 colonies, but everywhere. They could fight their wars and make their own alliances. The player could order them to break alliance or start a war, but that would increase liberty desire. I would also add a button to change the governor (assuming it's later in the game and we have governors, not vassals) to lower LD.
Obviously, they could rebel or just prioritize their own interests. I know that PDX said we will be encouraged to create vassals, but that's not a law of men but of physics. There are no telephones in the period; the guy that rules locally can create their own policies.