I'm having fun creating the alternate history for the "Empire of Mirrors" universe, a Steampunk fantasy where the British Empire never fell and is ruled by two queens: a rejuvenated Queen Victoria and the Clockwork Empress, the Queen of the Faerie Folk. In this universe, the Great War ended in 1917 in a truce between Britain and Prussia while America stayed out of the war.
France ends up a client state of Great Britain while the Netherlands become part of the Prussian Empire, and a state of Cold War exists between the two superpowers while anarchists in France and Russia attempt to overthrow them.
The Second World War will be fought between the Soviet Union and an unlikely alliance between Britain and Prussia while the Nazi Party fails and its members are jailed for treason and sedition. Adolf Hitler dies in 1951 from syphilis, an unknown painter and failed architect.
The war in the Pacific continues much as it does in our universe, except that it continues until 1947 and Japan is invaded by the United States since there is no atomic bomb. More than a million American soldiers lose their lives to the Japanese and their steam-powered mecha before they surrender, and only because the United States reveals the truth about the atomic bomb--in this universe, the spirit world is very close to the material plane, and detonating a nuclear device would disrupt dragon lines across Japan and Southeast Asia while enraged kami and angry ghosts would ravage the mortal realm.
(this was discovered in 1945 when the atomic bomb tests rend a hole in the fabric of the spirit world and the American Southwest becomes a wasteland of walking corpses and evil spirits for more than a generation)
The United States government detonates only one hydrogen bomb, which was at Bikini Atoll in the South Pacific; such was the destructive power of the explosion that it tore a rift between universes, releasing a monster the likes of which the world had never seen. Known as Gojira, the enraged titan ravaged across the Pacific Rim toward Japan, and only the combined forces of the United States Navy and the Japanese Mecha Corps succeeded in defeating it. Refusing to learn the lessons from the Americans, the U.S.S.R. detonated their own hydrogen bomb near the Arctic Circle, sending psychic shockwaves across the spirit world; the explosion awakened the monstrous Zmei, the legendary dragons from Russian mythology as well as Baba Yaga, sending the Russian Empire into chaos.
I love the visual texture of this universe: a mix of Gernsbach and Victorian styles with elements of art deco and art nouveau taking place in the 1920's, with blimps, airships, and dirigibles as well as biplanes and other early aircraft while in the streets are everything from steam-powered automobiles to horse-drawn carriages. The population of London is a bizarre mixture of Human, Faerie, and half-breed Changelings where Low Faeries occupy the working class while the House of Lords is slowly taken over by the Daoine Sidhe simply by virtue of outliving their mortal rivals.
It should be noted that during the Witch Hunts of the 1600's, Queen Mab and many Faeries of the Seelie Court escaped across the Atlantic to America, where they took up residence in the Appalachians; during the Revolution, more than a few of the Faerie Folk were happy to take up arms against the British to settle old scores.
When Sir Isaac Newton rediscovered the Laws of Magic in the late 1600's and wrote the famous Principia Magica, the Age of Enlightenment became a time of experimentation when the ancient laws of magic and alchemy could be organized and transformed into a science, and portals were opened to Faerie. Humans and Faerie Folk became friends once more, but most of the first wave of Faeries who traveled to Earth were of the Unseelie Court: Goblins, Hobgoblins, Redcaps, Trolls, Kobolds, Knockers, and Pooka. Members of the Seelie Court have since arrived in Europe, but they are greatly outnumbered by their opposite number.
As far as mortals are concerned, most humans in this alternate universe are much like their counterparts, but with some exceptions: for example, Albert Einstein discovers that magic does not violate the Laws of Relativity, but subtly changes them in its presence while Sir Arthur Conan Doyle has become the special envoy of Queen Victoria to the faerie courts. Lewis Carroll wrote about the adventures of Alice Liddell in the Realm of Faerie, but wonders if she ever returned to her beloved Wonderland...