Similar to Dark souls, if we find special upgrade items throughout the game, it would be great if we could upgrade the weapon stats even more and add enchanted properties. At this point it could be done only in BG2 when you reach blacksmith in docks and Cespenar late in the game and with just particular items. What if we could do it more frequently with all weapons and up to +10? Special made weapons and armors would requires special upgrade materials. The forge could be found at blacksmiths in bigger towns or in important areas. Upgrade items should be well hidden and not easily obtained along with the risk and reward.
PS: Will this break D&D ruleset or it's within limits?