Hello. I have been convinced by the arguments on this thread that annexation (inside Europe or your own religion) should be much more difficult than in EU1. Here's one way of doing it, following suggestions of sean9898 and Alatriste. I would like to hear what those with more knowledge of history than me think the details should be.
Why not add a relation "claim to throne" (CT) that can hold between countries in the same way as a CB.
Any annexation without a CT leads to unthinkable diplomatic, badboy and stability penalties. Any annexation gives a CB to every country with a CT on the country annexed.
CTs are gained by
-Events. In particular a dynasty dying out gives CTs to all of a country's neighbours. This might mean physical neighbours, those of the same religion, or those with close diplomatic links (RMs).
-As a diplomatic option, in return for a large amount of money. This will not give a CT immediately, but only after thirty years/after the current king dies. This will only work for small countries with no other hope of survival.
-Neighbours can claim CTs when a country has been at -3 stability for more than eight years.
-You get automatic CTs on your revolters.
-Give CTs mutually as a super RM. Maybe such a situation should amplify future changes in relations, whether good or bad, since the stakes are higher.
And for extra complexity:
-You can gain a CT on a country if you can persuade sufficiently many of its neighbours to accept your claim, either through diplomacy or as an option in a peace. Alternatively, good relations with and a large donation to the papacy will do the trick. Again, this comes into force after thirty years. Maybe it could be modelled as a success in a dice roll against relations.
You can give up a CT to improve relations, or for money, or in a peace deal. This will be the main goal of a revolter in a war of independence. And so on.
I think that something like this would add to the gameplay, by requiring clever diplomacy for successful annexations, as well as brute force. For a rampaging game, convert to paganism or declare yourself head of your own church to opt out of the system and run riot.
Now for my on-topic suggestion for a possible EUII patch: unthinkable penalties for any annexation within your own community, unless you have a CB shield on the relevant capital. Combine this with manipulation of CB shields by events, and automatic revolt risks in conquered capitals. Possibly the penalties should decrease over the course of the game.