Yes that sounds great. Is it possible for it to be configurable though, for those that want the all the higher level creatures?
Kinda. You'll just want to alter the encounter zone which determines the maximum level to pull from (and hence, de-levels on a list that otherwise would have defaulted to a lower level).
Just been playing around with it in-game, and it works remarkably well. Just for the fun of it I set the percentage chance to 100% and at level 2 wasteland spawns were like a battle of the titans. I think a default chance of 8-10% might work out really well and add a bit of spice to the game. A hardcoe mode of 30% would be for crazy crazy people (you know who you are).
The un-leveling has been applied to most, but not all, spawns -- areas where it shouldn't apply or it wouldn't make sense to apply (for eg quest-related). I have quite a long to-do list but I might release a beta this weekend for people to play with that includes all the updates for RC6 so far. This include this unleveling feature and a fix for the health-regenerating tripping Deathclaws (itself a fix for a vanilla bug).
It seems I am getting back into modding *shakes fist at Fallout 3*.