Yep, I can see this mechanism being incredibly annoying at high difficulty.
It's strange because it does not sound like ciV for which all things that happened where good and everything was about opportunity costs.
Well, Shirk said in the RockPaperShotgun interview that it
is going to wok this way and you
really won't want to change your ideology. Well, I guess you'll choose wisely according to your neighbours or keep culture and happiness up.
Actually there are multiple examples throughout history, where one culture/city/territory has been assimilated into a neighboring culture. One of the more recent examples is Northern Ireland, who chose to split with the Republic of Ireland in 1922. You could make a comparison to the border disputes of India and Pakistan too.
Or if you see the European Union as some kind of empire you can use the recent expansions as an example when it gobbled up the western part of the former Eastern Bloc.
So, next matter of speculation: ideologies and victory conditions. There will be three branches for every ideology, each tailored for a specific victory, and there are four victory conditios meaning every ideology will have one victory that they won't be able to efficiently pursue.
I can't really see an Autocracy civilization pursue a cultural or a diplomatic victory. A cultural victory would probably make a bit more sense than a diplomatic one. Conquerors often loot subdued countries for works of art and build monuments in their capitals, but we know from the interview that Freedom and Order both have culture centric branches and aesthetics would dictate that the excluded victory condition is different for each ideology. I also can't really think of any ideology that would exclude a science vitory, except maybe Autocracy again, after all the Nazis did do a fine job of driving many of their scientists into their enemies' arms, and I don't know what victory type could be excluded for Order civs except culture, but it's been stated that
Order will ahve a culture branch.