I don't know enough about how NPCs will work to have a strong opinion one way or another, but I'm not against the idea of enslaved NPCs. It seems like it would be especially appropriate on Tartaros.
If an NPC slave system is being considered, I want to put forth Life is Feudal's slave system for having player slaves. In Life is Feudal, when an enemy player is knocked out, an "enslave" ability can be used on them. This ability kills the player and creates an NPC slave with the player's stats and appearance. The enslaved player respawns and can play as usual but their title is forced to "Slave" for a few days, and the enslave ability doesn't work on them while they have this title. The NPC slave lives for 10 days before expiring.
I thought it was a pretty interesting system even if the game itself wasn't so great.