Empires with high imperial decay should actually break at some point.

[zijistark]

Currently, they [empires] just get above a certain threshold of fucked from the flat maluses of high Imperial Decay, and then quite anticlimactically, they just suck hard at everything for the rest of the game, but nothing really incentivizes their eventual destruction / being torn limb from limb by external aggressors / etc.

I'd really like to get some brainstorming feedback on the various things we could do to provide a more a mechanical (i.e., not just flat malus modifiers) decline of such an empire. This is separate from the question of interesting ways that AI empires could be handicapped or, more likely, "saved" from spiraling ID.

One idea — not a complete solution at all, but it could contribute — would be to actually give neighbors of the declined empire (above some high threshold of decay) a conquest CB vs. that empire. It'd probably be in the flavor of Imperial Reconquest (not the tiers or anything like that -- just a simple, non-holy-war CB that targets a duchy and vassalizes everyone under it). Certainly, the empire is already weak AF, so the AI may indeed actually use this CB to take more and more bites out of the declined empire. Between this and other CBs, this would promote eventual empire disintegration. It's also certainly more satisfying to see the empire start losing territory.

The only "problem" I have with this idea is that I slightly struggle to see "this land is under the wrongful control of a completely incompetent, probably decadent, ineffective, and doomed empire" as a rightful casus belli for war in all contexts (not that it has to be available in all contexts, though). However, the "doomed" part does help, as one could interpret that as ultimately the simplest form of claiming land by right of conquest -- there is effectively not even an entity ruling over that land.

Other things we could do might involve border vassals simply setting themselves independent gradually (just as over-vassal-limit succession works, except not necessarily on succession and nothing to do with vassal limit). [ The top liege still gets a Revassalize CB against them, but good luck using it effectively without armies. ] My problem with this is that it could lead to a lot more border gore, and it could leave a lot of minors out in the cold where they may be promptly eaten up by other CBs anyway, and every other kind of CB is going to unland and break vassal-liege structure for far more (ex-)vassals of the declined empire moreso than the proposed.

More ideas on how to really break a declined empire in fairly straightforward ways are welcome. I'll later submit a sister issue to this on how we can build better systems for saving empires from total decline (one spoiler: a dedicated decay overthrow faction would be one nice thing -- turmoil from within at the poor governance prompts a true overthrow, which also lowers the empire's decay significantly).