“Modern post- colonial” nations, such as Canada, Colombia and Australia, just to mention some “modern post-colonial” nations that appeared in Civ6, could easily have lots of paths to be formed.
Just as an example, you could have all this paths to get to a Colombia/Gran Colombia civ
Rome>Castile/Spain>Colombia
Greece>Castile/Spain>Colombia
Goths/Visigoths>Castile/Spain>Colombia
Goths/Visigoths>HRE>Colombia (due to Klein-Venedig)
Celtiberians/Celts>Castile/Spain>Colombia
Carthage/Phoenicia>Castile/Spain>Colombia
I don’t think it would be appropriate to have Native nations switching into Argentina or Colombia. Maybe the Aztec>Mexico and Inca>Peru ones could work, but something like Mapuche>Argentina or Muisca>Colombia would be a bit weird. (Though I think it would be awesome to have the Muisca in Civ7)
The Natives were generally against the independence of New Granada/Colombia and Argentina. They fought on the side of the Spanish against Simón Bolívar, for instance. The governments of Colombia and Argentina later continued attacking the Natives and placing them on religious boarding schools to eliminate their culture. At least in Colombia, the 1 million remaining Natives, out of a 50 million country, generally don’t identify with Colombia much.
While Nahua and Quechua peoples are quite numerous in both Mexico and Peru and those countries “accepted” their Native populations much earlier than the other post-colonial nations. They became part of their historical narrative much earlier.
That said, Mexico would have all the previously mentioned paths that Colombia could have, plus the Aztec.
Canada could also be formed by all the paths that would take you to Exploration Era Normandy, Francia/Medieval France or Medieval England.
Moreover, it would be awesome to have a Castile Exploration Era civ, focused on exploration, conquest and religion, and a Spain Modern Era civ, focused on culture and tourism. After all, Spain began to exist as a unified kingdom after the Nueva Planta Decrees of 1714, when the Bourbons began ruling. Before that, and during the Habsburg era, it worked as a series of personal unions with separate Cortes (akin to Parliaments), with Castile being the most prominent.