... well, I had a faction pop up
within a month of starting. Old Gods start date, whilst under attack by infidels.
UPDATE: It got worse. I'm not even a year into the game, only three vassals have a negative opinion, and I have a 135% Strength faction clamouring for autonomy.
This is in August. One year in, it's hovering at the 200% mark. But apparently the vassals are favouring the status quo according to an event?
The autonomy faction is expected to fill-out quickly. Since it is a neutral faction (merely a collection of your nobles that favor their general autonomy within the realm but want to stay in the realm) with a "mood consensus" mechanic that may lead to positive, neutral, negative, and no action taken toward / presented to you, the faction strength is only a worry when you or your laws are not sufficiently popular with your vassals. If you've ever played CK2+, it's like a more general princely faction. Unfortunately, there's nothing to do about the "Dangerous Factions" alert that pops-up other than to mod the alert icon to say "Factions" and have the tooltip for the alert reflect said possible neutrality (however, non-autonomy factions can still form, and they are always "dangerous," so one still wants the pop-up alert with the list and relative strengths to show). [By "popup," I mean the alert at the top of the screen.]
OPINION 1: Uprisings are now frustratingly difficult, verging on nigh impossible. Even if you crush every sizeable rebel army, that gives enough time for some of your holdings to be sieged--which leads to playing whack-a-mole and having to split your army into as many groups as possible, because the garrisons won't reinforce in time to stop the 29-man levies that the rebels keep raising from taking them back. The result is spending years putting down a rebellion you outnumber 10-to-1, haemorrhaging funds, because you need to have a force on nigh on every loyalist province just to stop the warscore from plummeting due to distant vassals' castles being constantly sieged and reclaimed. And then, due to the siege issue, you have to start sieging the rebels... despite their total lack of military power by this point.
Maybe it would be easier if I had 30 retinue units. As it is...
Though I forgot White Peace meant the law didn't change.
... still, the inclusion of nigh on every vassal you possess has... problems.
Your problem applies to any other strong revolt. The fact that the revolts are pretty much all strong now, if they happen, does present more of a challenge. Also, using your spymaster to coerce strong vassals (suppose you have a couple king / really strong vassals in your mix) out of the Autonomy faction is harder than managing a much more limited set of vassals in a traditional specific-type-of-revolt faction. Same with using your chancellor to improve diplomatic relations with faction members: they're still going to remain faction members probably at 100 opinion, but they'll just vote more positively.
The Autonomy faction in combination with patch 2.0 de jure tier liege levies, as it is, will basically make it well nigh impossible to manage an empire (or worse, a realm that straddles two de jure empires de facto) with anything but the most decentralized set of laws. That might be essentially a WAD outcome, or it may punish the king/emperor a little too severely for having a lot of vassals. It certainly promotes the arguments for using kings as vassals once you're an empire to a new level of relevancy (the debate, at least), as then you just might have a shot at being able to diplomatically manage your vassals well enough to avoid faction wars even with title revocation. OTOH, kings tend to really like their autonomy, although I don't know whether this is included in Meneth's weighting for faction membership or, most importantly, for voting. IMO, there is nothing about being a king under an emperor that is different from being a superduke under an emperor or a duke/superduke under a king, so they should not be intrinsically more difficult just because they're king vassals.
Which brings to mind a question, Meneth: I remember you saying that counts got 1 vote, dukes got 2, and kings got 3. Shouldn't it be something like 1 / 2 / 4 or 1 / 2 / 5? Or, better yet, you could scale the vote weight with the vassal's realm_size tier, regardless of title rank. This would make vote outcomes more fair and make it more clear/accurate which vassals to prioritize in your diplomatic undertakings. Since the faction action, if there is one, will include all faction members (let us assume all vassals), it is potentially very backwards to have the aggregate votes of certain relative nobodies that are more pissed-off (because they're rightly not a priority) trump the relative power of vassals that the liege can afford to favor. The realm_size tiers could break down into integer weights if you want, although that's not necessary to reduce complexity. I realize that going off realm_size requires considerably more lines of code to implement due to CK2's poor variable support (did I mention that EU4 1.4 will have full inter- and intra-scope variable comparison along with proper support for all arithmetic operators? hot damn. wrong game, though!), but since your weighting and mood summation is necessarily already too complex to be something that is actually directly visible to users, I don't see any reason to shy away from adding more complexity to vote weighting, given that it actually would make a big difference.
SUGGESTION: ... is it possible to disable the 'vassals have a meeting event' whilst one uprising over law is ongoing? It didn't happen, but the fact that I got an announcement a few months into the conflict that they were favouring the status quo raises the possibility of wars' constantly starting.
This is valid. There should be the equivalent of a do_not_disturb flag on the faction leader if there is an unresolved faction action ongoing. [Probably more like checking for any war in which the faction leader is an attacker, though not necessarily the primary attacker due to faction leaders changing upon death, and the liege is the primary defender.]