Base raid idea: stolen idols. You can enable or disable different raids by stealing enemy's idols.

So, why not a stolen idol mechanic?
Let's say that sometimes you could find a shrine somewhere that could randomly spawns at any scavenger camp, or hollow hall, or shroud infested zone, or whatever.
You kill them and loot the idol, and store in a box.
Now if you take it out and place it in your base, it spawn raids from time to time (and only when there are players logged in, to avoid destruction of your base when there are no players).
You can find and place more than one and of different kinds. Each of them will spawn raids. Enemies spawn away from the base, and they should try to reach the stolen idol. Either time based or number of waves.f they get it and take it out of the base boundaries, you lose the object and stop getting raids from them. You can find again the object later and steal it.
If the object is put back into a box, it stops spawning raids.
Raid enemy range level should be linked to the ones on the place where the idol was stolen.