1) Napoleon does better (as mentioned)
2) British Empire builds/leads (probably the latter) a thalassocracy, ruling the oceans and controlling almost all international trade. Smaller nations (e.g. Latin American nations) and less powerful ones (like the various Indian states and China) are economically forced to join because of the trade advantages. Eventually even France and Russia (the last holdouts) read the writing on the wall.
3) A more competent German dictator WINS WWII, harnesses nationalist feelings in the Ukraine, Baltics and Central Asia to depose the Soviets, take France, ally with Spain and Italy, take Britain. Tough to do, but possible with a WWI PoD
4) China industrializes, engages in maritime expansion, the wealth that maritime trade brings in increases the power of the merchants/middle class. Philippines, Taiwan, Vietnam and Korea are all IN FACT, not just in the minds of the Chinese Emperor, subject nations. By the time Europeans arrive in the Far East with any power, the Chinese have solidified their position, and are moving into e.g. Thailand and Burma and Indonesia. Australia is colonized by the Chinese instead of the British. By 1800, there's rail from Beijing to Vladivostok, and south to Saigon, and heading to Rangoon. Chinese colonization turns the west coast of North America Chinese. By 2000, every nation has to pay at least nominal tribute to the Chinese Empire.
5) the Ottomans do a lot better. ITTL every time they tried to siege Vienna, say, they spent most of the campaigning season bringing troops and logistics north. Here, they build big local bases, so they can move forward from them, not from Istanbul. They take Vienna and Poland. They keep control of the Muslim world from Tunisia to the Iranian border.
They take Persia. and later expand into the 'stans.
Industrialization, whether it's invented there or in Britain/Belgium, takes off and factories and railways spring up. The railways knit the growing empire together, and when oil is discovered and becomes useful, the Ottomans control all the Middle East supply.
Sultanates in South East Asia (in OTL's Malaya and Indonesia) ask for Ottoman help against European colonizers.
By 1900, the Ottomans have subject nations from the Philippines through to half of Africa.
Not sure how they take Western Europe and the Americas...
6) a nasty European power (e.g. Tsarist or Soviet Russia, or non-Nazi Germany) takes over most of Europe. The rest of the world has to unite in an American dominated alliance to crush them, and say 'never again'. This alliance ends up being de jure or de facto a World Government. (think of CalBear's AngloAmerican Nazi war, or a Drakaverse scenario where the Draka lose.)
7) Jonathan Edelstein's Malê Rising, except the World Court (I forget the name, ?Consistory?) ends up with even more power, and can be viewed as a world government, if you squint at it in the right way.