I'm running a sandbox with several adventure locations that have been visited, but not entirely cleared, and ones that have been left open for new residents to arrive.
I'm looking for a method for determining how these adventure locations change when no player character visits the place for some time.
After player characters have left a location, I restock it using wandering monsters and random treasure charts; this approach works fine.
How to handle longer PC absence, given that restocking only works once and only makes sense in locations with fairly vibrant ecology or factions that can grow or have reserves?
Ideally, the method would allow enough room for GM to consider the specific situation in their setting, would not require too much time or detailed attention to several locations, but would still be fairly rigid (e.g. "roll d6 per location, on a 1 make some changes" leaves too much to GM whim and provides too little inspiration).
Edit: There are several factions with clear goals, and several without clear goals, since they have not been seen in actual play too often. Ideally, the method would take decisions about which factions act, when and how successfully, out of my hands. I already have some of this in random encounter charts: One can encounter creatures from other nearby adventure locations in wilderness and in adventure locations, and these are easy to interpret as scouts, warbands, raiders, slavers, diplomats, or whatever.