If adding NPC camps to the open world would break the new surge mechanic too much then how about just making those thralls that do come through the wild surge portals wander around after they fall out of the sky instead of just standing around for a limited time?
Don’t tether them to a spawn point so they can just keep walking.
It’s more believable that they would start to wander and wonder about what just happened to them after all and you could end up with roving bands of NPC running into player builds and such.
They would fight animals, walk into undead camps and cause bedlam… it would be great to watch or jump into battle with.
If there is a worry about them cluttering up the map over time then have them die off after 24 hours or something.
It would make the island feel more alive overall.