A random thought occured to me today for a really simple way to handle magic item creation for items with plus values. Allow a player to trade a level from their character, to create a magical item (using their own life force) of a plus value equal to the number of levels being traded.
Example; a level 3 character could make a sword + 2 and go back to being a level 1 character, or a sword +1 and go back to being level 2.
Another example: a level 20 character could drop to 15 and get a sword +5.
Other, more special items (that do not have + values) would have to be handled by a different system, or the DM could flat out disallow the creation of them, perhaps all special magical items were made by an ancient civilisation and the art of creating them has now been lost.