This occurred to me earlier and I'm really surprised I can't remember it ever coming up on the board before, it seems like a really obvious one to do. Well, maybe it has and I just haven't noticed--if so, apologies.
What if John Wilkes Booth had failed in his assassination attempt and Abraham Lincoln served out his full second term as US president until 1868? How would this affect post-ACW attitudes to the south and Reconstruction? I tend to think it could increase enmity relative to OTL as the man the Confederates regarded as the tyrant would still be ruling over them for years after their rebellion was crushed, rather than being replaced with a succession of Republicans they might have hated but did not have such a personal connexion with.