Lots of good ideas in this thread! I'd like to add these ideas:
* "Number of Great Power Allies" diplo malus should scale with nation size so the penaly is less for small nations (maybe down to 0% for OPMs?).
* Diplo relation slots could somehow scale with size, maybe a 1-5p minor would get +50% more slots, rounded down.
This would make small nations more likely to survive by having strong alliances AND some perment vassals/marches.
I would love for it to be more viable to not expand, but for this to be fun we would have to get more stuff to manage during peace time, and for that I have no ideas except colonisation which is already in the game but ofc not available to all small nations. Edit: I like Zaks ideas for this!
I'll start a new topic with all of my ideas as soon as I return home. Hope PDX would work something playable out of them.