Maybe the imperial decay-mechanic employed in HIP could be a solution? Or at least a solid starting point / inspiration for a new mechanism? It's basically a negative factor that decreases parameter's like levy-size, tax-income and some other things by every succession / lost war / ... . It can be countered by winning war's and performing some awesome, prestigious deeds. But in the long run it's getting harder and harder to keep the imperical decay factor low or at the status quo level. It fucks over weak rulers and provides a challenge for capable one's. Overall it tones down the unplausible and ahistorical CK2 tendency to foster blobby giants in eternity. You can also define starting level's for the imperical-decay factor per realm.

Naturally, it only affects realms of the empire-level.

For example (and if I recall correctly), in HIP's 1066 start, the ERE starts with a much higher imperical decay-factor than the HRE, simply to mirror how old it already was at the game's start (and presumably how rotten / decayed / the moral of its leaders, bureaucracy and so forth). This prevents the ERE from blobbing in almost every game. It happens from time to time, given they have capable ruler's, but one will also see them fall apart every now and then.

I'd like to provide you with the dev diary, which HIP's EMF wrote back then, when introducing that mechanic, but I can't find it. I'll keep searching.