Not sure how plausible this is but, I can see a Northern rather than Southern secession in ~1860ish. The casus belli would be if Taney's Supreme Court declared that it was unconstitutional for states to prohibit slavery and emancipate slaves within their borders, and it was indeed widely believed that he was preparing to do so as soon he was given a relevant case (namely, Lemmon v. New York was working its way up through the courts.) This was preempted by the secession in OTL.
So, the POD would be something like this: Enough voters switch their votes that Lincoln loses California, Oregon, Illinois, and Indiana, leaving him without an electoral majority (this needs both a more unified Democratic party - or at least a more pragmatic one that saw union tickets in more crucial states, and some actual vote switching - Lincoln got majorities in enough states that a fusion opposition wouldn't be enough.) Since nobody has an electoral majority, this throws the race to the House of Representatives.
Let's assume that the states which were majority vacant in OTL (due to secession) would be controlled by Democrats ITTL. That is, TN, VA, AL, AR, FL, GA, LA, MS, NC, SC, TX (11 states in total.) If the other states have the same delegations as OTL, and they vote along party lines, the tally is:
Rep - 14
Dem - 15
Unionists - 4
Deadlock - 1 (CT is tied between Democrats and Republicans)
Which leaves no one with a majority of the vote. Given this, I think it's reasonable for either Stephen Douglas or John Bell to be selected for president as a compromise candidate (depending on how many electoral votes they have ITTL.) So, the US has another few years of an ineffectual Buchanan-esque presidency, where tensions ratchet up higher and higher.
And then, around 1862-1863 (2-3 years after the Court of Appeals heard the case), Roger B. Taney issues down a decision in Lemmon v. New York, ruling that it is unconstitutional for a state to prohibit slavery. The decision further polarizes the country. An attempt at compromise (involving a constitutional amendment which gives states the right to prohibit slavery within their territories) fails, and before long, the northern states are seceding. I really don't think there'd be significant bloodshed in this scenario, but hard to say.