Interesting idea, I found myself imagining how the code would look as I was reading it. It sounds possible. The npc info is stored statically in the database, but you can edit their properties temporarily just like you can in-game.
I'm not sure why you would want to do this though. Why not just have a level 1-10 area, 10-20, etc... To save yourself work and just recycle the same zones?
Consider this:
What if two level 5's are in Crushbone having an epic battle with orc pawns and a level 20 enters the zone to kill d'vinn for his dragoon dirk? It averages to be level 10 and now they are fighting a few level 10 orc pawns. Now generalize this problem. Unless you find a solution to that then I don't really like the idea except maybe as a challenge to implement it.
Also this should be moved to Development::Feature Requests or Development:
evelopment.