I seem to remember from the planescape campaign that the magical enchantments would be reduced by one for every shift from the plane the weapon was created. So if you were in the Outlands with a +5 sword created in Faerun, it would only be +4. If that same weapon was then in Greyhawk, it would be +3.
If we did this with the new content, it could make the new content more fun.
It would mean any weapon from Forgotten Realms would be bad in Eberron.