Whta might have been interesting was if there was a Kennedy/Johnson effect. Kennedy talked about civil rights but it was Johnson who really got it accomplished, in part by invoking Kennedy’s memory.
Reagan talked about cutting government spending but wasn’t able to accomplish it. Suppose Reagan had been killed and Bush had pushed serious spending cuts through Congress as part of the Reagan legacy?
There is a treatment of this in Harry Harrison’s A Transatlantic Tunnel, Hurrah!, where the American Revolution failed and The British American colonies expanded little, respecting the rights of the Native Americans, who became allies. France and Spain still had their footholds on the continent. The World Wars didn’t happen. In 1973, a descendant of I K Brunel plans a railway tunnel under the Atlantic.
Alexander the Great doesn’t die by poison/disease (whichever it really was), thus hopefully allowing him to live long enough to properly establish a lasting empire.
I think the sexual revolutions was more dependent on the Pill than Vietnam. But Nixon would certainly avoid mistakes in the Bay of Pigs and overthrowing Diem in Vietnam and without being seen as inexperienced, Khruschev won’t place missiles on Cuba.
It still would have been a bloody legal mess, with Navarre invaded and split in half (the northern half under French protection, the southern ruled by Aragon) but still keeping her own laws, four separate legal systems in Castille alone, another half-dozen in Aragon, plus Portugal’s. Singly-ruled, yes: unified, nowhere near.
If I could prevent an event, see the change in history, and then undo the change, I would prevent the asteroid impact which created the Chicxulub crater.
Tsarevich Alexei gets Empress Alexandra’s other X chromosome and is not born with hemophilia. Rasputin never enters the picture. I have absolutely no clue what this would do to history, since Nicholas II was an imbecile before he came along, but it would be interesting to find out.