Alright, we can consider a proven method for loading/unloading simulation zones — used in many action games, including GTA IV: “Bridges” or transitional sctructures. While you’re on one side of the “bridge”, agents on the other side remain in a non-active simulation layer.
These transitional structures can take the form of physical bridges, highways, or other connective zones. By stitching together multiple segments this way, and leaving only baked geometric LODs in the distance, you won’t compromise immersion in any way. In return, you get seamless locations — like a residential district, a suburban estate, a Hollywood studio, a concert arena, a hospital near Bliss Bay, and so on.
You could take a short drive via taxi/bus or your car while the next simulation layer quietly comes online, or wait a second as you move through a teleport. Either way, it’s still Bliss Bay, it’s still physical presence, and it’s still a lightweight simulation.