Make it like PSO2’s current grinding/enchancement system. Use other weapons to increase grind/enhance level. As well as having a cap.
With the grind exp increasing from rarity, same weapon type, same weapon name, and grind value of the fodder.
With it also having a higher cost the higher the rarity (or level), as well as having a exponential growth the higher the grind value to promote making grind fodders and potentially reducing end cost.
Of course the higher the rarity/level the better the exp growth it has, but it requires more grind exp to increase it.
This would also give more reason to use all of those random weapon drops for something and it would even promote something new and good to sell in the market at all levels.
IMC can even sell exp boosters to further increase the grind exp gain from the fodders or do it like PSO2 and have a “great success” function which increases grind exp gain and sell % booster to that.
It would even give more reason to make lower level grinded/enhanced weapons instead of saving all the anvils for your special endgame weapon. And armor pieces as well.
Though in PSO2’s case, max attack increase from grinding/enhancement is only +35% of the original zero grind weapon. Instead of whatever absurd values ToS has. Though, honestly, reducing the gained attack from grinding/enhancing would help tremendously for balancing, make attack stats from STR and INT more relevant, and would actually show some sense and skill for parts of the game instead of just one shot bosses. Of course, it would also piss off a lot of people because “MUH DMG!!”