The top de jure empire capitol only matters for the ERE or the Latin Empire (for which the entire Empire's legitimacy revolved around the possession of the Queen City). For other empires that aren't exempt (e.g., the Horde empires are exempt until they convert to another religion), it's simply a matter of slipping below a realm size of 100 holdings or 150 holdings when playing with SWMH. For reference, Byzantium and the HRE have over 4x that at start. Charlemagne's minimum realm size for its dynamic empire titles shall also be congruent with these minimums signifying major loss of legitimacy as a proper Empire.
Nice, sounds like a good mechanic to take care of those rump state empires that pop up occasionally (and the "give kingdom" in return is a nice tweak to it).
A couple things I've noticed from my last game:
Killing children via plot is terribly easy, still, much like Vanilla. Has this been tweaked at all in PB or EMF? It's pretty easy to trim dynasty trees exactly how you want over long term by just murdering children non-stop.
On the "murder plot" note, I've noticed that (like Vanilla, again) the "drunkard blabs" event is incredibly common. I know in Vanilla it was not connected to traits in any way, and worry this might still be the case. Additionally, that event is required to be confirmed by the plot leader before negative effects kick in; a savvy player can simply cancel the plot before confirming the event, and skips all negative repercussions (even plot target doesn't get angry!). Any way to move negative effects of this to the event for the character which triggers the leak? (Or at least remove dependencies on active plot to fire negative repercussions)
Additionally, the ease with which Primogeniture is established throughout Europe seems a bit off. Starting from Old Gods, it's not unusual at all to see all Karling Kingdoms at Primogeniture well before 1000CE, and over 75% of vassals throughout Christian realm having moved to Primogeniture. While I like that HIP removes the "keep Crown Authority at Medium, vassals never leave Gavelkind" strategy of Vanilla, as that was a bit too easy to control vassals indefinitely, it's very odd to see Primogeniture established in centrally weak states just because the ruler inherited a decent amount of dynastic prestige and gets a good long reign modifier. Perhaps tying this to tech in some way could help (or something akin to Charlemagne's planned "Centralization" slider law, which is also tied to tech).
I don't think it should be a hard lock based on laws (as in Vanilla), but ease of installing Primo succession should definitely be more strongly related in some way to the centralized power of a realm, instead of just prestige.
EDIT: A second check shows that Kingom's must have at least some level of increased crown laws to enact Primogeniture, but I still feel these are far too loose (Any ONE of title revocation or ruler's peace laws? Single generation accomplishment), based on Karling Kingdoms ALL going to Primo so early and easily. Also, Dukes and below being omitted from being tied to Kingdom Laws is also fairly jarring.
EDIT 2: Noticed you have a "plots" issue already listed on github, and the need for reform there. Reading through it reminded me of another plot issue I ran into; I repeatedly had issues with unlanded courtiers of mine (usually wrong religion marriage partners for dynasty members) go on plot murder sprees of my spymasters. All my spymasters were unlanded dynasty members, and these courtiers would never get flagged for just imprisonment or anything, despite being found out on successful completion nearly every time. Infuriating, as I would need to take Tyranny to resolve the issue, and they were consistently trimming my family tree of some of it's most gifted Intrigue individuals (I train all female dynasty members to Intrigue, usually). Also have no idea how a wrong religion/culture character was successfully plot killing dynasty spymasters usually over 20 intrigue each, but that's entirely separate.
Wow, this turned into a wallpost, sorry about that.