aren't there ansestral weapon rules in oriential adventures, where you spend xp and gold to awaken the next ability in a weapon? I don't own the book but I borrowed it from my GM for a bit. I seem to remember that. You may not want to have a wizard/weapon smith change your sword but you can "awaken" new abilities in it that were susposibility already there.
I think I like the idea of designing the sword ahead of time and then awaking it's abilities. you could combine these two by using the rules from OA but setting ahead of time what the abilities are.