I recall discussing that a few times in the past, but similarities to AOE-3 were likely viewed negatively, which might be why it wasn’t even considered. It’s a shame, because the concept of minor factions and mercenaries adds great flexibility and depth to the game; it allows for the inclusion of cultures that wouldn’t make sense as full-fledged civilizations, and it offers a lot of replayability.
But my enthusiasm for this game has already died, so I’m looking forward to this kind of thing in future titles.