I personally favour a reorganisation along early Roman imperial lines for rUK. This would have the distinct advantage of moving the administrative capital to Colchester, easing the pressure on house prices in London as civil servants, politicians, and lobbyists move to East Anglia.
I lived in the US for seven years. It's an interesting old place. Ninja is right that there's a lot of constant tension between the state and federal level. However, even though that and the culture wars exist, it's hard to see any particular issue or group of issues getting enough traction to spark a series of secessions. I just can't see any serious appetite for it. An awful lot of the Republicans who produce things like the shutdowns do so because they fundamentally don't believe in having any form of government at all. So it's hard to see them agitating for any kind of alternative sovereign state based on their own states, because they just don't believe in the State at all. Particularly not one that is governed by a democratically elected Black man. They are fundamentally not interested in democracy at all.
The only potentially destabilising thing I can see on the horizon would be if there were big enough demographic changes in the southwest. If you had a majority Hispanic population in say TX, NM, AZ and southern California, you'd have a group of states that were geographically linked, shared common ethnic, linguistic and cultural ties, and might be politically cohesive and fairly politically distinct from the rest of the US. And if you could take TX and southern California with you, you'd have a pretty strong basis for a powerful economy. However I think that's still a long shot. Hispanic Americans are generally really, really happy to be Americans and I don't see this happening any time soon.
People have also been talking for a while about splitting California up into a bunch of smaller states. There's currently a proposal going around to split it into six states. However the likeliest constitutional change to happen in the near future would be statehood for DC. Even that though is not super likely - it is way down the agenda for almost everyone except DC residents, and the Republicans almost certainly wouldn't let it happen because on current demographics DC would only ever vote Democrat.
It's interesting because as Ninja says there are some fairly deep politico-cultural divisions in the US, more so than in Canada, I would say, and the federal government doesn't seem to work as effectively as in Canada. Yet it is Canada that has most recently gotten closest to having a province secede. The American Civil War casts a very long shadow indeed I think ...
Applefalling You are closer to the mark than you think ...