You might look at Europe, for a set of countries that can and do exist in the modern world, with total area and population not dissimilar to the USA. They have mostly formed a voluntary association called the European Union (EU), but the constitution of the EU explicitly gives any state the right to leave the EU. (In contrast the USA fought a civil war to establish the opposite). My own country, the UK, is now in the process of leaving the EU. Interesting times, for sure. It'll take the next decade to find out where this leads.
Anyway, I don't know enough about the USA to answer this question, but since this is worldbuilding: imagine that in a near future alternate reality, Washington DC was suddenly struck by a large meteorite (total death and destruction). There's just enough advance warning that the natural cause of this disaster is known and WW3 is not triggered, but not enough warning to get anyone in DC to safety. Not even the president.
Might the states decide that since the federal government of the USA had effectively ceased to exist, it was time to decide that the country had likewise, and each state was now sovereign? If not, add whatever plot elements are needed to ensure that they do so decide. Then ask, which states would inevitably find sufficient common cause with their neighbours that they decide to join together into single bigger units?
You might also dig out that copy of Heinlein's Friday which paints a picture of a near-future Balkanised North America. The interstellar travel parts of the plot are far less plausible than the alternative non-USA it portrays.
One other point: "real" countries need coastlines so they can trade freely. Landlocked countries are always vulnerable to being squeezed by their neighbours, leading either to hostilities or to strong-armed mergers. Bear this in mind if you decide to draw a map of a fragmented North America. It's mostly because my country is an island, and the rest of Europe is not, that the whole Brexit thing has happened. Which other countries are feeling least happy with the EU? I'd suggest Spain, Italy and Greece. All have sea on three sides and a mountain range on the fourth. Coincidence? I don't think so.
One last datum: Singapore. It's a city-state. Unlike the Vatican City or Monaco, it's a "proper" country. (Also, technically at least, an island).