No, I mean if you search for all the factions in campaigns that could be added as civs, isn’t there more than two possible combo per existing civ without a campaign?
For instance, we could add the Saxons and Irish to the game, as they are present in York (for both) and Hastings + Barbarossa 1 (for the Saxons only), and maybe other but I don’t have all of them in mind right now. Which means we already have 6 possible combos instead of one for the Vikings alone, and there are probably countless other possibilities. Why should I specifically pick Welsh+Dutch?
I probably don’t know enough about the Mayas, but I know they fought against the Spanish (which might be a little bit redundant with Montezuma though) and I’m pretty sure they faced some Nahua people at some point. I would also be surprised if none of them ever fought against the Zapotecs or Mixtecs.