I disagree (of course). This is too much like the sort of random event that frustrates people. I realize you're not saying it should be random, but it would
appear random, and that's enough.
Legionary, I hadn't realized the point about infinite resources enabling large empires. It's a good one.
I am against ideas that bias the game directly against large empires or empires that have been stable for a long time. It just seems like a mean hack to have code like:
if (cityCount > 50)
revolt();
or:
if (now - lastGovChange > 50)
revolt();
Any disadvantage that large empires or old governments have should be a natural consequence of the same rules that apply in every situation. Diminishing returns, as Aussie Lurker mentioned, would be one way of doing that.
Regarding the quality of citizens, I think that should basically be
education. It's hard to educate your citizens. Educated citizens are more productive.
I wonder how city specialization will work. I think it would be best if it was an acquired trait, rather than a directed one; i.e., nurtured like a garden rather than commanded by fiat. That would make it more precious when attained and also harder to switch from one specialization to another. Think Damascus steel or Venetian glass. Furthermore, if the city is conquered, it should retain its specialization.
When Constantinople fell, there was barely any empire left. The Ottomans already were on both sides of the Bosporus and had Greece.
Nor should it make you automatically weak. After all, size didn't destroy the British Empire, at least not in any obvious, direct way. Really, that didn't happen with the Roman Empire either. It should be more than just size; cultural strength, military strength, degree of interconnectedness (due to roads, harbors, etc., according to
effective distance), how long you've held the various cities, religion, ethnicity, and other factors should all be incorporated. It should be possible to balance them all well enough to sustain a large empire. Possible, but not easy (assuming you're playing on an appropriate difficulty level).