Supercharge colonialism.
Quinine anti-malarial properties get discovered and adopted en masse at the very beginning of the 18th century, around a century ahead of OTL, enabling the colonization of sub-Saharan Africa and Southeast Asia now that malaria is substantially less of an issue. Peru fails to lock down their cinchona seeds, so the Dutch manage to get their hands on some and began making quinine plantations in Indonesia, thus ensuring a solid global supply of antimalarials.
Trekboers stumble upon the Witwatersrand gold deposits (or the Kimberly diamonds), triggering a frenzy for the African El Dorado, with people chasing historical accounts of the Mutapa and Timbuktu.
Shenanigans involving the Holy Roman Empire result in Frederick the Great turning away from looking at sugar extraction research, delaying its introduction and increasing the demand for cane sugar and thus slave labor.
European powers invest in colonial endeavors instead of domestic development, slowing down industrialization. In addition, human capital flees from Europe in the hope of striking fortunes in Africa and/or fleeing persecution in Europe, draining that professional class key to industrialization.
Various polities in Asia and Africa also gain access to quinine, sparking a new era of large-scale jungle warfare and colonialism. The Qing make a more earnest push into northern Vietnam and exert stronger control over Taiwan, resulting in bloodier, sometimes genocidal campaigns.
When many newly claimed territories end up not containing mineral wealth, Europeans turn towards plantations, cultivating sugar and spices all across Africa fueled by massive slave plantations rivalling those of the Caribbean. The slave trade surges in size, and when combined with other factors, such as the introduction of productive but resource-intensive crops from the Columbia Exchange and the proliferation of modern weaponry, results in devastating warfare across Africa, like the Mfecane, but on steroids.
The economic prosperity of slavery is shortly followed by people looking for a moral justification for slavery, which when coinciding with the Enlightenment, results in the early formation of scientific racism. These two factors result in abolitionist societies being significantly weaker than OTL and abolition being substantially delayed.
The outlet valve of emigration to the colonies also reduces pressure for democratic reform in the metropole, so absolute monarchies remain much stronger and face less of a risk of revolution.
European great power conflicts, due to a larger colonial presence, draw in not only their colonies, but also native allies, resulting in large-scale wars that often last far longer in the colonies.
Global trade surges, but that also means a more interconnected world with many more disease vectors. The third bubonic plague wave hits earlier in the 19th century and expands farther and faster than its counterparts, and half a century earlier, meaning there's substantially less advancements in medicine, hygiene, and epidemiology to mitigate its effects. We get ~10% death rates worldwide.
Weaker imperial metropoles from Europe to East Africa to China that have spent way too much money in their colonies and conquests while industrializing less, face substantial unrest. The Qing collapse earlier, with various warlords and a Taiping-esque radicalized theocracy fighting a vicious civil war to become the new dynasty, while colonial revolutions break out across the world, often with polarized populations where there are equally large shares of loyalists, neutrals, and rebels. Some succeed, while others are brutally repressed, but many of these independent colonies end up continuing their overlord's colonialism, just under a new flag.
The HRE fractures during this time, but no hegemon emerges and the great powers are too preoccupied to jointly establish something like the German Confederation, resulting in a bunch of isolated city-states at the mercy of their larger neighbors. The HRE ends up being partitioned in various subsidiary client states and continues to be a national battleground repeatedly trodden over by great powers, despite undercurrents of pan-Germanism.
Also, malarial resistance to quinine develops, causing even more deaths in the tropical world around the same time the bubonic plague hits. Disease depopulation and protracted warfare result in entire regions collapsing, with the largest form of organization being a depopulated city-states, with some societies returning to tribal societies. When colonial overlords return, they do so with an iron grip that is much less lenient on their weakened subjects, enabled by industrialization finally occurring and allowing stronger power projection from the metropole.
So by the time the 20th century rolls around, the world is a depopulated, backwards, and unstable shadow of its OTL self.
