What if there was a way to craft legendary artifact sets that could be slotted into sockets on equipment? I was thinking it would be cool if you could craft a "Golden Shamrock" set which offers increased gold and gemstone drop rates. This would be a buff that works like the global buff found in ANB events, and it would stack the same way. Similarly, you could craft a "Stone of Enlightenment" set which offers a global experience bonus.

Why Sets?
I think it would be fun if the bonus was only offered as a 7 piece set bonus. This means you would need to craft 7 pieces and slot all 7 of them into sockets in order to receive the full bonus. This would force people to choose between 1 set or the other, and it would also increase the demand for sockets on equipment.

The Cost?
Considering that these would be powerful set bonuses, I would hope that they would be difficult to craft. I would think each piece should cost 10 Brilliant gemstones of each type as well as some gold to craft. These buffs are currently unique to events, so perhaps the recipe could call for a Celestial Essence (only available as event rewards) as well. This would prevent people from crafting these sets on ANB event heroes.

These numbers are what I would go with, however the developers are probably better at determining the best balance. Maybe they would prefer a 6 piece set or a different crafting cost. If the developers like this idea, I'll leave it up to them to determine the amounts by which these sets would raise your exp, gold and gemstone drops.

I've kept the above idea simple, but I was also considering having each piece require a unique/new type of material. The purpose here is to have this set require you to achieve certain things in the game. 1 unique type of material would be awarded when you complete each achievement. The achievements would require things such as earning all 5 stars for every story level on each difficulty, completing trial level 125, reaching Champion Level 1,000, unlocking all stash tabs, unlocking all character slots and creating 5 characters to fill those slots, etc. The new materials could be named "Virtue of ___", with each achievement awarding a different virtue. This will give people reasons to experience certain aspects of the game that they might might not have experienced before.