If I'm not mistaken, the OP mentioned another scenario possibility- in which a post-apocalyptic North America suffers central government collapse and a reversion to "feudal" local warlords. It's unfortunate that this was shot down so quickly- mainly because it is intriguing as a historic analogue. Much of what the real-life Middle Ages was (especially the Early Middle Ages) was a result of a pseudo "post-apocalyptic" society in which former central imperial governments (namely the Roman Empire) and Kings had lost most of their power and in which people grouped together under local lords for protection in what was essentially a glorified gang-system. Having some kind of near-future scenario in which nuclear war between China and the West (let's say, over the DPRK) causes total societal and governmental collapse, leading to abandonment of the cities and a rural society with no central government, which thus begins (rather naturally in a case of "convergent evolution") to resemble so-called "feudal" Europe after about a hundred years. As well, like in historical Europe, powerful lords could in theory try and "re-form" the "old empires" (IE, Canada, US, Mexico etc as creatable King titles, the various states and provinces as creatable Duke titles).
People will probably flame this, as tends to happen on Paradox forums, but let me state that I am a stickler against anachronisms and hollywoodisms...which is exactly why this sort of scenario would appeal to me. Mainly because the player wouldn't have to worry about things going wonky or ahistorical...it's in the future and not historical at all. However, as it seems that you have already decided to do a 1066 alt-history, I maybe (yeah, maybe) could do a total-conversion sub-mod on the same or similar map which takes your 1066 alt-history and makes it a 2200 alt-history. Again, don't flame me (people WILL), nobody'd be forcing you to play it if theoretically it was made.
For my part, I favored this route when I began this thread. I could see the mod taking either path; we have quite some time before the game is released. However, I see the following issues or concerns that would need to be addressed while exploring this option:
1.) Backstory:
a.) What caused the societal collapse/decline?
b.) What implications, if any, does the original cause have for the present world? In other words, if nuclear war is the reason, what would this mean for agriculture?
2.) Modern Technologies
a.) How would we treat modern technologies?
b.) Would they vanish somehow? Would people slowly forget how to repair or maintain them?
c.) Would we adapt them to the CKII game in terms of warfare?
3.) The World
a.) Would you simulate interaction with others off the map, i.e. Europe or South America? How?
b.) How would you treat major game features like the Papacy or the HRE, assuming its in this time?
c.) How would the political systems of the past be analagous to the future? In other words, would it be reasonable to think we would regress into inheritance of title/land? Would republics vanish on a sufficient scale to make this a workable CKII mod?
I could go on, but I'm getting tired. All these issues are intriguing to think about, and there are several advantages to this type of mod. For me, an obvious is there will be little need to rename places; we could simply adapt the current names with different spellings. Another advantage: we all certainly have a firmer grasp on our own time than the past.
On the other hand, over the past week or so I think I have identified the chief issue with the present scenario we have been thinking about, and it would largely be a non-issue in a post-apocalyptic NA. We have been exploring the issue of location for likely/probable settlements in NA for European transplants. This issue is nearly moot with this scenario; you would still likely see population migrations to warmer climates depending on the nature of the decline/collapse.
In the end, both ideas are worth exploring. I would be willing to support whichever you, the CKII community, believe would be the most fun to develop.