Following this out of interest. South America tends not to receive a lot of attention in althistory, and when it does, it's the same set of 19th century clichés: the Empire of Brazil, the Peruvian-Bolivian Confederation, Gran Colombia, and British Patagonia. Two ideas I have for post-1900 both involve replacing models of governance. Virtually every post-Iberian country on the continent currently uses a fully presidential system, much like in the US. However, this was not always the case.
Uruguay
used to be a directorial republic, much like Switzerland, from 1918 to 1933 and again from 1952 to 1967; during this time, a nine-member body jointly served as head of state and government. Six members were selected from the winning party in the prior election, while the other three were taken from the second-largest party. In the first directorial system, the Directory co-existed with a President. This seems to have worked well, but a power struggle between the President and the Directory led to the former couping the latter in 1933. The second directorial system was structured differently. There was no longer a separate president; rather, the office was a
primus inter pares presidency and rotated between the six members from the majority party. It was apparently ineffective, with the majorities often bickering with one another and the president lacking control over the ministries, so it was abolished in a 1966 referendum.
Meanwhile, Chile
was a parliamentary republic until 1925, though it deviated from the Westminster system (and from most parliamentary republics in general) by giving the ceremonial president a bit more power (though not enough to turn the country into a parliamentary republic with an executive presidency, like South Africa, or an assembly-independent republic, like Suriname). Specifically, the President was able to dissolve the legislature if they had a parliamentary majority in support of such a motion. In most parliamentary republics, the president cannot do this, and instead the PM must dissolve the government and call a snap election. In the so-called "quasi-parliamentary" or "pseudo-parliamentary" system used in Chile, the president could completely bypass this process. In theory, this means that if the President and Parliament work together to do so, they can sack and replace the PM without holding elections. Or at least, that's how I personally interpreted Chile's deviant version of the parliamentary system. It's not very clearly explained how it works, so I could be wrong; corrections are appreciated and encouraged.
Either way, those are my two PoD suggestions for you: you could tweak politics and structure so that the directorial system in Uruguay and parliamentary system in Chile aren't ineffective (and thus replaced by presidential systems), and subsequently are retained by these countries to the present.