I've been thinking further on the idea of making the majority of magic items in my OD&D campaign unique. Here’s a draft mechanic I’ll be testing out in my game for weapons and armor.
Arms and armor, if finely crafted by an expert, MAY have a +1 bonus based purely on the extraordinary nature of the craftsmanship that went into their forging. (This will depend on the cost of the item… such work requires the full attention of the smith and will cost no less than ten times the usual amount for the item.) When a character begins to use such an item (whether or not it has a bonus), I'll keep track of the combat experience gained during its use, with the item earning an amount equal to 1/4 of that earned by the character. (If the item is intelligent, such as a magical sword, it will gain experience at 1/2 the rate of its wielder, like an NPC, including experience earned for treasure.) The trigger events can take place any time AFTER the requisite amount of experience has been earned. Magical Attacks are “survived” only if both the item and the wielder make their save. Heroic Actions must be combat-related, appropriate to the level of the wielder, and played out in game.
These charts show how these items can level up:
In addition, these items CAN earn experience from treasure in one way: the treasure in question must be spent specifically to non-magically enhance the item in question. For example, decorating the item with gems or precious metals would count, but treasure spent to pay for further enchantments would not!
It should also be noted that magic arms and armor found as treasure must also become attuned to their wielders. Those powers that work by default (for example combat bonuses) will become available one at a time for every 100 experience the item earns with the wielder, while those that require a command word will have to be discovered through character research and action! Magical items found can be improved over time through experience using the above tables, but experience earned must be great enough to have “created” the item in the first place (so raising a found +2 sword to +3 would require it to first earn 200 to fully attune to the wielder, and then 3000 more experience and a Heroic Action .
Final thoughts: This method will be very slow, since the item earns experience far more slowly than a player character would. This is NOT meant to replace the usual magic item research and creation process by spell-casters, but to provide an in-game alternate method based on the actions of the characters!