I posted this on the D3 Bliz forums, but i'm posting it here for visibility and discussion. I think this can add a very fun risk to the game and help advance characters into further GRs:
As a means to improve items and add a rewarding risk to the game, allow the mystic/cube to upgrade legendaries or only ancient legendaries.
What does this mean?
Using an NPC such as the Mystic or Cube, give equipment the option to be upgraded with a chance of failure.
What happens to an upgraded item?
If successful, every base stat of the item should be increased -- Not only would every base stat be increased but all rolls of those stats would reroll in the new range as well.
Ex: An ancient Yangs bow has the following default stats at level 0:
At level 1 which may have 95-100% chance of upgrade
If not successful, break the item. Make it unusable. I'd say destroy it; however, some people may want to show how high they've gotten something before it broke on them.
The leveling system doesn't have to be capped, but the chance of success should decrease with each level.
What kind of regents would it use?
Either new crafting regents can be implemented or it can require all bounty items/crafting items. The items needed to upgrade should increase per level of the equipment.
If there is a max upgrade level, an item that has been maxed can be signified by a border such as ancient items already are. (Maybe a purple/diamond/plat border)
This crafting system has existed in other games that i've played and it adds a very fun element. One thing i do NOT suggest is adding items to repair the item. This kind of ruins the risk.