According to Wikipedia, the 2 most popular alternative history subgenres can be summed up as “Axis Victory” and “CSA & the U.S. Civil War”. The latter can be easily seen on this website – I bet if we were to categorize the threads on the Before 1900 subform by topic, the overwhelming majority would be about the CSA and the U.S. civil war.
So here is your challenge: come up with a scenario where U.S. undergoes a civil war or a war of secession but one that does not revolve around “the south” seceding. I’d prefer if you base this on real OTL movements that could have potentially lead to a civil war scenario, but you can come up with your own ATL movements.