I've been playing a few games from CM, and it seems inevitable that several large kingdoms and empires will form and simply remain in power for eternity. CK2 seems to be doing a worse and worse job of modeling the explosive nature of European politics during this time, especially in regards to large kingdoms such as France or the HRE. I even had a Pictland grow and stay alive throughout a 400-year game, it took nearly everything needed to form Brittania, and stalwartly refused to evolve into Scotland or England. The Karling titles seem equally stubborn, if an early king manages to unite enough titles to form an empire, Western Europe is essentially Karling (or another big dynasty) for the next several hundred years.
For starters, Pictland
is Scotland, just with a different name because it's controlled by Picts, not Scots.
And yes, if someone creates an Empire (Andalusia+a few bits, France+Aquitaine, the HRE) they tend to stay stable rather than shatter. That's kind of the point of building an empire, if it holds together past one ruler, it's probably stable.
It shouldn't be this way. Crusader Kings 2 is a game of story and events, where anything can, and should, happen. I'd like to see some event chains that can lead to the dissolution of a large empire/kingdom, whether that means a kingdom hemorrhages lands or collapses in on itself. I'd also like to see more successful independence factions within large empires and kingdoms. It should be HARD to keep a kingdom together, especially as crown authority grows (as every king eventually attempts to do). As it stands now, only Elective Gravelkind results in any kind of kingdom-breaking events upon succession, but such a mechanic makes sense even with early feudal powers.
So anything should happen... Except for being able to build and maintain a stable empire without deliberate efforts by the game to punish you for being successful?
Independence revolts should be difficult, unless they're large scale, and that requires them to have a reason to be large scale rather than just "Country X is too powerful, let's make 2/3 of his land revolt".
England had multiple wars based around Crown Authority or Claims. Not once did it have a serious "everything north of the Humber wants to be independent!" situation. Wales went to war a few times to break free of England's domain, but lost, largely due to scale. In general the wars tended not to be "I want to be king of an independent X", but rather "I want to be king of X
and Y".
I'm very much enjoying CM and how CK2 has grown since its creation. I own all the major DLC and I enjoy almost every minute I play, but I think this aspect could use some very real improvement. In addition, this makes it more enjoyable for the player, there's nothing more satisfying than seeing your enemy spontaneously break apart after a stroke of bad luck or an unruly faction. And its satisfyingly frustrating to have your own hard work to hold together an empire be shattered by a series of unfortunate events. Such is life in medieval times.
You might think it satisfyingly frustrating to have an empire you've built up shattered by unfortunate events. I disagree, especially when they're events that I cannot influence or control, and that have been piled into the game purely because I've become an Empire, or picked up a second Kingdom title, or worse still I'm fine at 149 holdings in the Empire, but as soon as I hit 150 I get bad events to break me apart. Even worse would be being fine at 50 holdings, but degraded to a Duke at 49 - even though I'm in a war to get those provinces back, and I'm now going to have to pay through the nose to get my kingdom back, and worse still the now non-de jure areas are revolting.