Another possibility: Washington is assassinated while leading the militia against the Whiskey Rebellion. He never crossed the Appalachians himself, but I could imagine a deluded fanatic coming east to kill him. It would have to be a lone fanatic - the large majority of the rebels surrendered at the rumor of the militia coming against them - but that could have huge effects. President Adams is likely to be far less forgiving than Washington, who pardoned the few rebel leaders who were actually put on trial. Adams, backed up by a populace roaring for Western blood, will likely try to root out all the rebels throughout the West. In response, I can see many rebels fleeing west, maybe joining up with the Indians, or (more likely) trying to pull off Burr's plan of seizing Louisiana early. They just might succeed. Or, even if they fail, there's going to be significant Western-Eastern resentment, which might have large effects down the line.