You may have declared Baseballfan's scenario as a winner but I'm not done mulling this over.
Pierce was a surprising compromise candidate in 1852. If he doesn't get the nomination then then he probably never will. This also means that any really big POD before 1852 probably dooms Pierce to an also-ran.
Bleeding Kansas and Ostead were not of his design, even if you change Pierce himself then you probably still get these two events.
So far, any attempt to make Pierce a great president have had to reverse Pierce's pro-south and pro-slavery opinions, I'm not sure how likely that is. Is it possible to have him keep his pro-south stance and still be great?
Don't sell him short: he was likable, fairly intelligent, handsome, well connected, ambitious, and a war leader - we've had worse individuals in the White House (he was defiantly the wrong man for the time, however). I'm surprised that we have a fairly interesting man who was president of the US at a crucial time but the most recent biography of him was published over 45 years ago.
Going from thin information on the internet, It's remarkable how much of a drag his wife was: she forced him away from Washington DC, she hated his political ambitions, he had to run for the nomination behind his wife's back, she was convinced Pierce was doomed by God, she spent every day of the first two years in the White House writing letters to her dead son --- what a drag! This would drive a lot of men to drink.
I'm thinking that to make Pierce great we need to bump off his wife sometime before 1852 (then again Lincoln's wife was no winner either).
The train accident that killed his son, how about it kills his wife instead and spares Benny?