well in some games (i'm thinking of AOM specifically) it's possible to program map scripts and randomly include units and buildings which belong to the Gaia player and are neutral/aggressive to all players indiscrimniately. for example, the inclusion of Settlements in AOM that you can build new Town Centers over, the random bands of Skraelings on the Vinlandsaga map, and Bandit Migdols (Egyptian Fortresses) on some maps which are specifically spawned near gold mines, adding a little challenge to early expansion.for teh purposes of gameplay, regular player units (like hoplites) could be treated like aggressive animals when controlled by the "gaia" player, meaning that they'll ignore your units for a while but will attack if you hang around for too long. i know the animla comparison could have racist connotations, but i think that's just the easiest way to handle this. basically, a gaia-controlled hoplite represents a native of a given region who sees one of your gatherers and thinks "Hmm...I don't like the looks of that guy" but leaves the guy alone for a while, but eventually gets uncomfortable and thinks "that guy has overstayed his welcome. time to kill 'im!" and attacks. as for actually "converting" a minifaction, i'd say a new interface would be needed, perhaps a second diplomacy window applying only to minifactions that you've built an Outpost next to, allowing you to give and receive tributes with the tribe and, after you tribute them a collective amount of resources (let's say 1000 altogether from all three resources, in any combination) which then allows you to train their mercenary units at your Barracks, Fortress, Embassy, etc.. going on the idea of basing the first minifactions on units already included, those mercenary units could be specifically banned for training by their "home" civs because doing it otherwise would be superfluous.