My thinking is that all the free states (except Oregon and California) in 1860 would secede. This makes 16, although if you count New England as a single state, it's 11, just like the CSA had in OTL. It might be useful to have New England united into a single state, as then the slave states would have a numerical advantage over the free states in the Senate.
My current best idea for a POD is having a guy like Roger Williams come along and found a denomination of Christianity in New England which is liberal and friendly to American Indians but eschews technology in favor of a more "natural" lifestyle. Through the power of selective butterflies, the colonies of New England unite into a single colony with this religion (a name for the religion escapes me) as the prevalent faith among its populace. The religion would become very popular in the Northern states but not so much in the South. History mostly proceeds as in OTL, what with the Revolutionary War, accession of western territories, War of 1812, Mexican-American War, etc. up to the 1850s or so. The main difference between OTL and the ATL is that the new religion has stunted the growth -- both in population and industry -- of the Northern states relative to the Southern ones. As such, the pro-slavery Southern states hold majorities in both houses, resulting in the North's inability to pass anti-slavery legislation which would give blacks the same rights as whites. In this TL, the South would be in favor of centralized government and the North in favor of states' rights, which is the reverse of OTL, of course. Even though the South is more industrialized than the North, their peculiar institution remains largely intact, with slaves working both in the field and in the factories. The dominance of pro-slavery sentiment in the government has led to the spread of slavery throughout the western territories and states, unlike in OTL. This all sets the stage for a confrontation in 1860/61 in which the North attempts to secede from the rest of the US just as the South did in OTL. Civil war ensues.
I'm not sure if any of this is even remotely plausible, though. What does everyone think?