Well, one thing to remember is that, if you just make it harder to control far-flung provinces, or those that are culturally different, people will just stop playing once they hit that point.
A few people might be encouraged to press on just to overcome the increased difficulty, it think you might be missing the whole point of playing, which is to have fun. I think you need a solution that make it FUN to press on. To some extent, it's fun to overcome obsticles, but if those obsticles are dumb friction (higher unrest), it won't be fun. The solution to the increased difficulty needs to be creativity or skill, not just greater and greater investments of time and/or garrison troops. IMO at least, it has to be a puzzle to solve, not a grind to get through.
One suggestion I have is, rather than increasing the difficulty of holding what you've taken (the unrest problem), perhaps the difficulty should come from you being attacked by coalitions of smaller nations. And not the headless stacks like in RTW, but I'm talking battles where you have been attacked by 2-3 actual armies at the same time, in the same battle. Using MTW as an example, if I am the English and I hold Flanders, the Germans, French, and Danes should ally against me, and then attack me all in the same year, so that my stack in Flanders faces three armies in battle. THAT would be a tough fight.
Another way to make it more fun and interesting would be to have different nations actually field armies of different kinds of units, and use them in different ways. If the same army and tactics I used to crush the Germans won't work against the Polish or the Italians, that will encourage me to overcome that obsticle and beat them.
But again, I suggest these because overcoming challenges with my mind is what makes these kinds of games fun for me, while overcoming them through patience or just more of the same is not fun for me. Other people may have different motivations.
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