Just to pitch in about "hindering the succesfull player". Hopefully if you do integrate these gameplay features, you will make them optional in the game setup menu. But if you think about it: In the real world, especially during modern eras, if one nation is dominating it is more likely to to become even bigger, and stronger, rather than to fall for something like internal strife. I mean it would be quite hard to "simulate" things like fall of rome without "killing" the fun out gameplay.
In my previous game, which ended in the "too much money" bug, I actually had 3 sort of "national slumps" so to speak.
1. At some point criminals started to run rampant in my "empire", somewhere around medieval era I think, and caused a lot of "oh my god what is happening", but I prevailed with producing LE units, which of course didn't solve the problem in a day, but still I prevailed.
2. Somewhere around renaissance, once again I think, I hit economical problems where I was struggling to just to stay a float, now of course there insn't a real solution to this, but to "stay alert" about what buildings you are producing in cities, and so try to avoid adding to city maintenance. Once again I prevailed.
3. Then there is the pollution problem during modern era etc. once again can cause a lot of headaches, but even this "challenge" can be beaten. All these three "challenges" felt natural, and in hindsight, were sort of fun.
To halt "steam rolling nations" perhaps the focus should more on the diplomatic side. I don't know if the C2C still has, or ever had(at least one of the previous versios did), the U.N units. Perhaps more U.N and "nations of the world" involment in diplomacy, and pressure throught economical means. Perhaps trading could be more linked to national relations in overall, or perhaps you could gain boost to economy if you are part of U.N etc. And considering how much you guys have put work in to some many new features, the U.N/Apostolic palace referendum/directives could use an overhaul. Perhaps adding those prohibitions(alcohol, drugs etc.) to the list, which would force all members to abide by.
The A.I does need some work, as I have previously stated, as "they" intent to outright suicidal. Declaring war against player of 60+ cities, when they only have one. I understand that this was most likely caused by another A.I pressuring for them to go to war. Perhaps A.I players could actually form alliances against big empires, that is if they have bad relations with said empire. And maybe instead of declaring war on the player(suiciding), they could contact player saying "we need help nation A is trying force us to go to war with you" etc. One thing would also be nice. When you are singing a defence pact, or if you are willing to accept a nation at war as your vassal, that you could force, or intimidate, the nation that are in war with, to make peace with the said vassal.
Plus any addition to diplomacy, and "spy games", which allow you to affect things in the "shadows" would be a nice addition.
1. Change the "change civic" spy mission so that you can actually pick, which civic you want the player change to, instead of just offering the civics which you have active.
2. High cost, long term, spy mission where you can change one players attitude towards another, not just you.
3. Spies lobbying for your corporations, or perhaps even invoking things like prohibitions.
4. Spreading another nation culture, which would lead to city revolting or just flipping to said culture.
In my previous game I send a large group of criminal units to another continent, and started conquering another nations cities. Now this was more about farming slaves, too keep those cogs turning in my empire, but when I think about it was kind of like I was funding criminal organizations to wreck havok in another nation, and it was fun

. Also it would nice If you could send your LE units, into another nation to help them quell criminal, or revolutionary, waves(yes once again thinking about farming slaves

). Sure this could be exploited, but perhaps you would need to a permission from the other nation to come and help. Maybe A.I players could even ask for help. I did find an "criminally" easyway of farming slaves, but I won't say it so you can't fix it XD XD. Once again sorry for going on for so long.