An idea: Single player exclusive "mini civs"

Yes, AoE 3 and 4, since their launches and because the low number of civs, had campaign-exclusive civs, to make it more historically accurate…

Campaign-only civilization | Age of Empires Series Wiki | Fandom

In Age of Empires III and Age of Empires IV, campaign-only civilizations are those civilizations which are playable by the player or encountered as AI players in the campaign mode, but are not available in other game modes like skirmish, ranked, custom lobbies, etc., whether single-player or multi-player.

For the campaign-only civilizations in Age of Empires III, see Campaign-only civilization (Age of Empires III).

For the campaign-only civilizations in Age of Empires IV, see Campaign-only civilization (Age of Empires IV).

For the civilizations encountered only in the Historical Battles in Age of Empires III Definitive Edition, but not campaigns, see Historical Battle-only civilization.