I know this is pretty ASB.
Benedict Arnold conquers Quebec in the Revolution, then assists Patriots in Nova Scotia. British North America becomes entirely American after peace is made.
The Northwest Ordinance is able to ban all future slavery, due to Quebecois and Nova Scotian votes, and Thomas Jefferson's insistence.
The South rebels over tariffs. This evolves into a regionalist and slavery supporting war, which the South loses, but because of little industrialization, it lasts just as long as the OTL Civil War. The Union army gains much more military experience this way, also.
A US more tolerant of Catholics conquers all of Mexico instead of just the northern third. Meanwhile, more immigration is received from Catholic countries.
When gold is discovered in California, corporate miners outnumber Anglo individuals who decide to go there to strike it rich. In addition, Mexican miners arrive. Due to the interaction and competition between the four different groups, the richer mining companies are able to keep the Chinese safe. More immigration comes in and there is no Exclusion.
At this point the US has everything from Quebec to Chiapas, except Alaska.
The US launches an expedition to Japan to open up trade like OTL, but establish more of a presence. When Hokkaido secedes, it comes under American control.
Conflict between Spain and the US leads to the US invading their colonies. Cuba and the Philippines are conquered. Cuba becomes loyal, after the slaves are freed. While the Philippines are allowed relative autonomy, and the Chinese there provide an additional pro-American faction.
Haiti and the Dominican Republic either vote to join the US due to economic troubles, or the US invades to liberate the Dominican Republic and conquer Haiti.
The US cooperates with Britain to force the abolishment of slavery in Brazil. Brazil becomes an American satellite.
By now it's the late 19th century. Gold is discovered in Alaska. American settlers flood in. War begins with Russia, and the US invades the Russian Far East--it becomes a US territory.
In the Scramble for Africa, the US supports Liberian interests. Liberia gets into a war with France, and comes under French occupation. Afterward they become a US territory.
By 1900, the US is giant and diverse superpower. Revolutions in corrupt states in Central and South America, and corporate interests, lead to American interventions there. They become US territories and states rather than just puppets, because by now the US is significantly Catholic and mestizo.
The Qing dynasty begins to fall apart. The Chinese American community begins to lobby for the support of Chinese Republicans. With bases in Hokkaido and the Russian Far East the US invades southward, and from the sea. The Manchus are punished and made into a protectorate. Korea, Taiwan, and Hainan become territories. While the mainland is a US ally.
In Europe, a Great War occurs. The US supports Britain and France against Germany and gains Namibia and some Pacific colony.
The disruption of global trade caused by the Great War, in combination with a stock market crash, leads to a global economic depression. To distract the public from economic issues, the administration engages in militarist expansion elsewhere. A war of "liberation" is waged against the remaining states in South America and Japan, many of which are kingdoms or dictatorships. Since the US has evolved into a tolerant, partially Hispanic and Chinese nation, this is successful.
But this is alarming to the powers of Europe, who see the US as a severe disruption to the global balance of power--and the kingdoms of Europe see it as a disruption of their conservative rule. A European coalition invades the US aims to force them to release recently conquered nations.
But the great powers fail to notice the rising of communists or pseudo-fascists in Germany and Russia, who proceed to invade outward with ideas of irredentism. As a result the European invasion is repelled. And by now the economic hard times are over.
While Europeans kill each other, and emigrants from the continent arrive in the US, the US conquers European colonies in Africa and the Pacific.
By 1950, the US owns the Americas, most of Africa, Oceania, most of Indonesia, all of the Pacific including one mainland area Korea, and the Russian Far East.