Area memory is basically the idea that area retains the events of the past in memory.
Memory is limited to remembering N events of the same class.
Suggested ideas:
1. Monsters spawned gain resistances to most common past spells or area spawns different monsters with more levels/resistances.
2. Monsters spawned may avoid specific class of player due past memory.
3. Harvested resource locations don't spawn resource.
4. Chests/Treasure don't appear at same location again.
5. Modified terrain retain its shape.