[Suggestion] A realistic way to challenge imposing empire (Decadence, rebellion and decline)

Hello,

In most medieval/fantasy games I have seen over the last decade, I haven’t really seen something that have occurred for most of the major Empire of the world: the inevitable decline and decay of any imposing civilisation. Due to a numerous number of factors such as the complexity to manage, the impossibility to defend across hundred of miles,  the cultural difference and jealousy over different regions of a same dominion etc, it was practically impossible for any big civilisation avoid a decline, or decay.

So, I would like to have your opinion about putting an in game mechanism to add some “danger” of creating an imposing empire (+50 cities, +2500 soldiers etc) across few continents. It would become really interesting if some city could rebel and become independent faction, if some army in time of peace become thugs and begin to plunders our cities located far away, and any such mechanism that will inevitably cripple any mega sized empire. Even if magic can be involved (for few exceptional people across a whole nation) the communication in a medieval fantasy realm should be a real pain and the management of a multi continental empire across few continents should be a really difficult thing to maintain.

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