I've got an insane idea for the Holy Avenger weapon. Make it a bound to character scroll that when read provides a feat to upgrade the summoned weapons from Paladin's Holy Sword spell. Reading the scroll would not consume it (allowing TR's to re-read it at the appropriate level).
Downsides are that you'd have to summon it each time you log in. It can't be crafted with rituals or cannith crafting. Upsides though are pretty big. 1 item = works for all paladins. If you change your weapon type on your paladin no problem! Just summon the new weapon type. And if Turbine makes changes to the weapon in the future they don't have to worry about having problems with the old version of the item. Players just summon it again and get the new version right away.