Once you’ve cleared out the area around your homebase in static, conflict can become kind of rare, unless you build a deathmobile and go on the roam. This also means that there’s essentially no value whatever to be found spending your time digging in and fortifying - moats, walls, automated solar-powered .50 turrets, all are essentially window dressing.
So I would suggest that (with this option on,) zombies sometimes spawn and drift across the map, in much the same way that NPCs do. If the player intercepts one by bringing the reality bubble into contact with the zed - or if it wanders into the player’s reality bubble, it spawns in and becomes realized, sticking around as if it were a static spawn; if not, then it drifts away into the aether, never to be seen again.
Also, if the player has the Scout trait, then wandering zeds/monsters would appear on the map screen the way wandering NPCs do. This would let the player decide if they want to evade/intercept intentionally.
You could also have a wandering horde, which functions, well, much like a horde on Dynamc. But, if a wandering horde passes an area that has already been cleared, new monsters will be spawned-in, representing zeds and others which became separated from the pack for generic monster reasons, thus giving the player new challenges to deal with in previously-cleared territory.
Anyway, just some thoughts I had.