I don't know if you guys have played Guild Wars 2, but they have an excellent leveling solution. You're level scales down to the area that you're in. Even if you're level 20, you get scaled to level 10 in a level 10 area. All enemies in that area are around levels 5-10. However, even though your stats scale, you still have better equipment and skills a level 10 character wouldn't have. The weapon stats scale, but a level 20 sword is still much better than a level 10 sword in a level 10 area.
With this system, you don't have to make creatures appear in areas where they shouldn't. There's no need to have "a lvl 20 skeleton is an Arch Lich" in area where there was previously just normal skeletons.
After playing GW2 for a while, I find the system very well done. It makes lower level areas easy, but not a waste of time.