Without new architecture sets, the most natural additions would be 2-3 civs for Africa and 2-3 civs for Central/South America.
The Pachacutec campaign could really benefit a lot from some more South American civs e.g. Chimu which already have a great civ concept here:
and are featured in the Realms mod too:
Some obvious African options would be Swahili, Kanem Bornu, Benin, Somali, Nubians.
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