Non-expansionnist wars have not been incentivized in a while. I've suggested a myriad of changes in the past, such as
*Shorter travel times all around
*Lower unit morale penalties for being far from the realm
*Increased effectiveness of the mercenary unit setting
*Increased gold and food return from looting
*Bonuses for realms that fight a lot
*Recruitment center boosts for dense realms
*Reduced militia effectiveness
*Increased militia decay
I think it's good to tell people "you can't just expand forever, and turn 100% of the realm's resources into maintaining a bloated low-density size", with various mechanics like the rogue mechanics (which, actually, kinda does that, but at least it allows everyone to participate, unlike region control issues), but I think it would be fair to also encourage people in the sense of "now that you've reached a good density ratio, you can take advantage of it to attack other realms without needing expansion as a reward to be worth it".
