[Skywatcher]

Not sure if durablility is being built into the game but you could use different materials to make weapons last longer rather than being more capable. Being made from a harder material means a blade would hold an edge longer. If weapon enhancements were done by adding magic items to the weapon then these magic items could possibly be recovered when a blade ceased funtioning so major investments in rare items didn't get lost when a blade failed. As far as level limits goes, I like the method of making the character possess skill to use various enhancements and even to apply various enhancements.

For example:

A fighter would require the skill for adamantium or mithril weapons to merely use that type of weapon. Then in order to add a +2 enhancement to it he would have to have skill in "enhancing" weapons. In order to add ice damage to the weapon he would have to have skill in "enchanting" weapons. The more enhanced or enchanted the weapon would be the higher skill he would have to have in the weapon type in general.

Better material weapons could also allow more or better enhancements. If durability was included then skill could be gained in mantaining weapons to make them last longer as well. Just some thoughts.