Challenge: A World War in the Western Hemisphere

What I mean by this is instead of Europe and/or Asia being the focus of a World War, have it be South, or some bizarre way, North America.

Is it possible? This would probably require something like a 2nd US Civil War but hey, that isn't impossible.
Well you've got two perfectly formidable combatants right there, North America vs South America. Depends when do you want this war to happen?
Problem is, the USA could fight every nation in the Westenr Hemisphere and still win.
This a thousand times, need outsiders to make this close to even

European intervention will be a must, need a pre 1918 divergence point, something that causes an Anglo American Rupture, activation of war plan Red-Orange (UK and Japan both), have Brazil and Argentina face off on opposite sides, Mexico with the UK and a few others supporting the UK and US respectively and you get a pretty nasty war, not truly world in scale that will require bringing in more europeans which will shift the focus there
Not with a POD in the 20th century.

If you go back farther, you could have Mexico winning the Mexican-American War, keeping it around as a major power. Perhaps you would need a slightly earlier POD in their independence, in order to keep them from falling into a cycle of dictators and corruption. You could also keep together Gran Colombia, which gives you another major power in South America, and have Brazil develop better. Possibly you could have Argentina gain Chile and Uruguay in the War of Independence.

I was considering doing a tl where the Mexicans were able to get their stuff together and form a competent government right off...

That would have major implications for the Americans "manifest destiny" right off.