How Balkanized is Balkanized?
The Union contains the heartland of modern America; although the loss of California would hurt, it isn't essential.
There are strong geopolitical reasons for the US emerging as it did: substantial iron, coal, etc reserves, plentiful river valleys and a lengthy coastline with many harbors to ease commerce, some of the most fertile land on earth, but also massively underpopulated in comparison to the Old World- even today the US is still far less crowded than Europe or Japan, in many ways similar to Russia, i.e. a core region of highly developed and historically controlled territory and a vast sparsely populated periphery conquered gradually via settlers and state action. Moreover any state with a presence in the Ohio River valley is going to want to control the headwater of the Mississippi, the North is going to want to expand south into New Orleans for that reason alone and will only fail to do so if there is a peer competitor committed to resisting them... and, really, who would care to stop them? Likewise with the expansion to the Pacific, which was literally built into the Articles of Confederation as a manifest March to the Sea, and eventually paid dividends with Gilded Age entanglements in the Pacific, e.g. Hawaii, the Philippines etc.
For comparison look to the Russian interest in a warm water port (manifesting in interest in Ukraine, Constantinople, and ultimately the Baltic and Balkans) and the French obsession with the Rhineland and Wallonia... the latter only failed to remain with France as a result of nearly a millenium of resistance by Great Powers, ranging from Ottonian Germany to the Habsburgs and eventually Great Britain... what state emerging in North America would be capable of resisting the Union colossus?
One needs a PoD well in advance of the Declaration of Independence; alter the settler colony patterns of Britain, France, etc. to get a more varied colonial pattern on the eastern seaboard, except.. why would this happen, in an ATL, when it didn't OTL? England and France were comparative newcomers... Spain Look to what happened with New Amsterdam- the Dutch had a go of it, then eventually the British stole it and now it's New York. You need to stop that from happening. Cripple Britain, or otherwise give them competition at home and abroad... the Dutch are too busy worrying about first the Spanish and then the French (and unlike the Brits they have no English Channel to protect them...).
The key therefore would seem to be to cripple France and thus wank the Low Countries, either under the Dutch Republic or an alt!Hapsburg or Valois king of Lotharingia/Belgium. Alternately, prevent Spain from getting entangled in the Habsburg Valois Conflicts thus allowing them to expend more manpower overseas.
If whoever owns the Netherlands doesn't have to focus so much on Europe (which, if France is super weak, or if the Netherlands are under a pro-Spanish Habsburg cadet line, is possible) then they can perhaps focus more on overseas ventures.