Walker WorldA virus in the Pliocene Era wipes out Eohippus and all its relatives. The horse never survived to be the powerhouse to replace human power. Face it: a team of oxen just doesn't cut it when you want to go fast. Things in the Old World happened about the same as in our world, until the Roman Era. The barbarians never developed cavalry, and the Roman legion remained king. This just delayed the inevitable about 200 years. Rome fell due to internal decay. The desert tribes of the Arabian peninsula remained flea-scratching nomads, and Mohammed never started Islam, or, at least, it never caught on. The Crusades never happened. The Indian subcontinent retained much of its early culture well into the second millennia. No knights of chivalry. Countries are much more stable, since armies must travel at a foot soldier's pace. TL 4 prevails.
I've played in this world. It's Tekumel, only not. Oxen don't make very good riding mounts.
Assassin's WorldRonald Reagan dies from John Hinckley's bullet. George Bush is ushered in, appointing Alexander Haig as his V.P. Two years later, Bush is assassinated during the Presidential campaign, and Haig invokes martial law. A massive FBI investigation reveals Bush's killing to be the work of a conspiracy of crazed lone assassins. The public buys the obvious oxymoron. Haig lasts thirteen months before, you guessed it, another lone kook blows his head off, as well as that of the Speaker of the House. Control of the country falls to the Joint Chiefs of Staff, two of which are killed within weeks. The Surgeon-General, C. Evert Coup, issues a report explaining that this rash of insanity is caused by a new virus that has infected the population: Acquired Insane Dementia Syndrome or AIDS. Anyone can carry it and pass it along in large groups, like football stadiums, religious rallies, corporate stockholder meetings, etc. No symptoms appear, until one day the victim wakes up in a homicidal frenzy. Coup's news conference is disrupted when he and many reporters are killed by a suicide bomber. Multinational corporations and tour guides carry this plague to the rest of the world. Chaos ensues.
Ew, where was my brain when I came up with that? Whew.