Are you envisioning the next civil war will be simultaneous sieges of all the major cities in the US?

No, most likely the same North-South split, with a bit of interior, vs exterior element to it.

That seems implausible. Slavery is no longer the overt separation between the areas. Democrats in Atlanta agree with Democrats in Portland far more than they agree with Republicans in rural Georgia. There's no basis for a civil war. Revolution is the closest we'd get, but neither side has anywhere near enough power to suppress the entirety of the other.

Plus, when the shit hits the fan, the reasonable among us seem to suddenly conclude maybe we don't really care to fight about this. Yesterday is a classic example. These folks at the capitol like to think they were going to start the war and everyone else would join in. What really happens, of course, is that they find out how limited their numbers are, and that regular people will simply direct the police to squash the insurrection.

American socio-economic divide is very much geographical, unless the only poor people you know are urban homeless.

And this divide lines uncannily along the lines from centuries ago.