It just occurred to me that there may be a nice way to add a dynamic feature to the game through either DLC or a mod. Imagine an NPC, say Preston, tells you that "Reports are that Super Mutants have begun to gather around such and such place. They are up to something," From that point on, the number of enemies at the named location increases each day by a preset value, up to reaching a limit or until the player goes and clears them. If the player takes their sweet time, the accumulated horde can start marching on their own and hit local settlements or other faction encampments.

Continuing with in the same fashion, the idea may be applied to the settlement attack mechanic and/or to the enemy respawn mechanic: instead of a random attack on a place by mobs that materialize from thin air, the mutants/ghouls/raiders/new enemies/etc. may begin to accumulate at various random points and once critical mass is reached to start roaming,

This is a great idea! hopefully someone will mod it.