Random idea: Would it be possible to have a random NPC horde of "wildlings" periodically come and try to destroy your settlement? Like an event? They would always be at your level of weapon-types, but just show up and you'd have to defend/fight them. Like a tornado or fire incident. Just wondering.
That's a great coincidence! I've actually been working on a project with a very similar concept for over a year now.
My idea is based on a 'Nomad Invasion' mechanic. The player would place a specific decoration/object outside the settlement to 'call' these Nomads, which then triggers chaos or fires. To deal with this, the mod includes fortifications and Watchtowers with patrolling guards—similar to the existing mods that add guards who bring random items, but adapted for defense.
It's still a work in progress, but I wanted to share that this is exactly the direction I've been taking with my mod. It's cool to see others thinking along the same lines!