I have been slowly making progress with the "T-Life" global AI. Here's a quick GIF showing the NPC squads bustling around a non-active map, visualized with symbols:
(Sorry about the small size, Steam only allows 5MB max per file)
Currently, the NPC squads (each capsule) traverse to a destination, wait for some time, and then go home. If the destination is on another map, it'll spend some time in a "traveling" state before emerging there. And if they emerge on a map player is on, actual NPCs will spawn according to the data in memory. All the NPCs preserve their stats - name, identity, appearance, health, etc. as they move between maps, which means you'll be able to follow them around.
It's been a challenging feature to code because I have to build it upon an existing NPC infrastructure which wasn't initially made for AI free roaming. But it's coming together!