I have played one game as the germans till ~1750 on monarch. Even though i have won the few wars i have engaged in the stability of the empire is now "collapsing".
I think the major issue is as Zetetic Apparat pointed out, that when one of my neighbours collapsed i often got a city or 2, which greatly dragged my stability down.
Another thing is, that atleast in my game, all ai players seem to have severe stability problems, except the more or less isolated ones (inca, aztec). So far china, india, egypt, persia, russia, france, turkey, greece, rome have collapsed, and the rest is more or less on their way to it. Since i only have played one game, it could ofc have had more collapses than what will usually happen. But atleast for my taste it seems abit much. I think that if i tried all that i could to stay as stable as possible, i could propably win simply by outlasting.
Imo it feels wrong when citys join my realm and i take stability hits for it. It should imho be the opposite. (My citizens gets confirmed that my realm is a great place to live
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When a collapse happens, i often see the ai disbanding pretty much all the units in the citys. Is that a known issue?
In one situation greece collapsed and one of there citys reverted to me, i also conquered 2 others. But quickly after the reverted city and one of the conquered became part of the reappering turkish empire. I declared war on turkey and took back the 2 citys without much trouble, however all in all the series of events seemed to cost me a huge amount of stability. I tried to reload and let turkey have its freedom, but that didnt change anything stability vice (atleast that i could see). Russia also had atleast one city revert to turkish and declared war like me. Even though turkey was destroyed (again), the war also coursed the (fairly) big russian empire to collapse.
Perhaps give a stability gain if you declare war on a revolting state if you have some types of rule? (police state, monarchy stuff). And give a stability gain if you let them go if you have, say, democracy? I think this makes atleast some sense with regards to reality.
Another thing that i would really like, was to be able to recreate a country that had collapsed as my vassal, if i had some of its citys.
As it is now, i dont dare to conquer independent citys, since im afraid that if i do so, the fallen empire will be reappear, with a big stability hit for me as a consequence.
All in all its a great mod though, and i enjoy playing it alot. Its right up my alley. Keep up the good work.