Why not allow players to change the stats on an item to another defined set (e.g. knights to dire for example) using purchased Stat Transmutation stones?
We all have had drops that would have been great (e.g. ascended boxes) if only the stupid thing were the right stats for the characters we play.
Similarly, you could have Weapon Type Transmutation. This particularly helps players who are moving from one class to another. And why not? You will get more revenue from gems than you will get for selling gold to buy another item on TP.