You've already done the plausible ones...
....so here's mine....

The Constitutional Conventions fail so the US breaks up. Europe dissolves in revolutionary wars, several British heirs to the throne are dead/ill/too young/whatever. The butterfly effect kicks in and whoever becomes king isn't capable of keeping the Empire together.
Spain's South American colonies become independent, and somebody similar to Bolivar comes along. He's familiar with the American ideas of democracy, and British government (and other ideas as well). He wants to try something similar. He starts out by unifying Argentina, and making it a successful, working democracy,
He and his descendants make Argentina more powerful, dominating South America. North America is so disunited that there's no effective resistance. Argentine scientists are also exploring Antarctica, and Argentine explorers have discovered gold in California, Alaska and Australia.
By 1900 Argentina controls all of North and South America. Oceania was taken over in this period as well.
The Scramble for Africa never happened, and Argentina's medical researchers are able to enable Argentine explorers and traders to deal effectively with local African conditions. Argentine corporations soon control mining and development in West and Southern Africa, and by 1925 Africa is entirely under Argentina's rule.
China and Japan see Argentina as a major rival, so Argentine agents arrange a civil war in China, and a Japanese invasion of China. Unfortunately the adage, "Never fight a land war in Asia" applies to the Japanese as well. By 1930, both sides are weak and weary enough for Argentine corporate do-gooders to come in and take over.
There was a Communist Revolution in Russia, but it went so badly, all the Argentine agents had to do was assassinate the leader and let the resulting civil war go on for a while. Argentina was able to take over by 1940. Controlling the media was a major help here. Television and radio were common by then.
Argentina bribed enough Ottoman officials to control the selection of the next Sultan, who promptly abdicated in favor of an Argentine governor, and the Ottoman empire blinked out of existence on January 1, 1950.
By this time, Europe was starting to unify against the Argentine threat, but it was too late. The European-Argentine War lasted from 1950-1954. It lasted that long because the Argentines wanted to test some new weapons systems, and they didn't want it to last longer because they wanted to minimize European hatred of the Argentines.
South Asia was the last region to join the Argentine empire. The last holdout, Ceylon, joined on July 11, 1970.