This may prove somewhat interesting: let's see if we collectively can come up with one or more plausible (non-ASB) points of departure and timeline concepts for each president of the United States. NOTE: clichés and obvious PODs are to be avoided! That means that Lincoln's avoidance of assassination is out of bounds for this challenge, for example.
Now, I realize that for William Henry Harrison and James Garfield, given their short tenures, avoiding dying in office may be about the only POD available in either case. Still, it would be interesting to see what the real scholars might come up with otherwise.
One other note: yes, I'm well aware that William McKinley served in office until September 1901. But in most operational aspects, he was the last 19th century president; therefore, I'm making the end of his tenure the cutoff.
Have at it! This should be interesting.