To be honest non of those city's would be nearly as big as they are if they weren't on a major international border.I got three major city mergers on the border between Texas & Mexico.
El Paso & Ciudad Juarez
Laredo & Nuevo Laredo
Brownsville & Matamoros
It's hard to imagine a different border though. The current border is very logical, passing through all the bodies of water.
Unless you have Michigan be part of Canada and the border be on land.
If Brazil had not remained united, I find it fairly unlikely that the site of OTL!Brasilia would ever hold something significant. Its a super-dry planalt away from pretty much everything and everyone. Pretty much no one lived there at the time Brasilia was built. A more probable "inland capital" might be somewhere more defensive in Minas, like São João D'el Rey.
Without South America being divided between Spain and Portugal, I highly doubt most amazonian cities as we know would exist. Perhaps alternate cities in the same or similar sites. Belém, Macapá, Óbidos, Manaus, Salvaterra, etc. Say, a Spanish-only South America might only have forts built in the coastal area, to prevent foreigners from getting INTO the Amazon River, rather than how the Portuguese did, where they also had to fortify against both foreigners coming from the Atlantic AND Spanish invaders from the hinterlands of South America.
So, maybe Belém and Macapá, but not Óbidos or Manaus
Hmm, not sure about Óbidos and Manaus. Both of those are on pretty strategic spots; Manaus in the Negro-Amazon/Solimões river convergence, Óbidos on the narrowest point of the Amazon River('only' 2 km across). It makes sense building forts, which would grow on to become cities, on those places.
Windsor (Ontario) would become part of Detroit metropolitan area if the US-Canada border was further south.
In the USA, if Delaware did not exist or had significantly different borders, Wilmington would be even smaller than it is with no downtown and be even more of a suburb of Philadelphia.
You meant the consolidation, right? If so, then Brooklyn may be the largest city in the United States if Queens ended up in Brooklyn. New York City, however, would still be the core of the New York Metropolitan Area (likely known as New York-Brooklyn-Newark instead of New York-Newark-Jersey City).New York City might still be separate cities if 1989 had not occurred.