This idea requires a few changes to work out, so I’ll be starting with those 
Hideout
After completing the campaign, the Horadric Vault becomes the player hideout. Here, they can invite NPC crafters, and other players. The most important one for the crafting system is a yet unknown character, a Horadric Mage and a forge master, who tracked down the player after hearing of their deeds. They want to restore the order to its previous glory, and will serve the player as a mage blacksmith. They need a certain artifact to reinvigorate the forge, a certain Horadric Malus. They send the player on a quest to retrieve it, and when they do, the Horadric Forge is up and running.
Horadric Forge
Located in the Horadric Vaults, and operated by a new Horadric Mage NPC, it is used to craft unique items.
Horadric Library
The Vault is also home to the Horadric library. It contains esoteric knowledge about the world which the Horadric Mage NPC uses to figure out how to craft Unique item recipes.
Other hideout features
This is a player hideout, and they can “decorate” this place as they please. Doing side quests around the world can now reward NPC followers, traders, and crafters, whom the player can invite to their hideout. For example, after finishing the Ally of the Bear Tribe quest chain, the player can invite some of the clan to join them, and then hire them from the Hideout.
A fully decked out hideout would have one of each crafter NPC, a waypoint, more storage space, a few wardrobe stands where a loadout and gear can be saved, space to store and display trophies, a stable (near the entrance of the vaults). Since this is a hideout, no merchants would be present here, only a select few warriors and tradesmen.
Crafting Unique from Common
Players have to find recipes, which can be found anywhere regular unique items can be found. Once a recipe is obtained, it needs to be added to the Horadric Library in the Vault by right-clicking or “reading” it. Next time they visit the Horadric Vault, they will have access to the recipe.
To craft an item, players would need to collect unique crafting materials. When they choose the item they want to craft by talking to the Horadric Mage NPC in the Vaults, they are sent on a quest chain. This quest chain may involve battling World Bosses, doing PvP content, exploring Nightmare Dungeons, and collecting special materials, among other tasks.
Each recipe requires a base item, which needs to be a sacred or an ancestral white item. In most cases the Item Power of the base would determine the difficulty of the quest chain, and the final power of the item.
Nightmare dungeon tasks would for instance require collecting the unique materials from a boss or finding it somewhere in the dungeon itself. These materials would only appear in the dungeon if the level of the sigil is appropriate (determined by the item power of the base item, as mentioned before). For example, a 625 power item would require only doing a level 1 nightmare dungeon; a 725 power item might require completing a level 21 nightmare dungeon; and 820+ power item might require completing a level 100 nightmare dungeon.
A world boss task would require just killing the boss. No matter the world tier, they are always easy content, so it doesn’t make much sense limiting tier 3 bosses to sacred items, or world tier 4 bosses to ancestral.
Example 1:
For the first example we will use an Ancestral Staff item power 715.
Speaking with the Horadric Mage NPC starts a quest chain, and the player is tasked with collecting the following special crafting materials:
- Find the staff “head” piece: Kill the Tomb Lord in Ancient Reservoir and collect his skull.
Since the nightmare dungeon level needed for the skull to drop is based on the Item power of the base item, for our example a 715 item power staff would require a minimum of level 19. - Reinforce the staff with bark of the Wood Wraith: Collect 20 Wood Wraith bark.
- Infuse the staff with the essence of undeath: Collect the essence of undeath from the Wandering Death (World Boss).
The player would then bring all of the special crafting materials to the forge, and forging would commence. But oh, no, something is not right. All of the souls that the Tomb Lord trapped over the years have been given strength by the essence of undeath. They are all released, and the player needs to defeat 5 waves of spawning enemies in high density - each a batch of Skeletons, Zombies, Ghosts, Vampires, and the Drowned. They are 2 levels above the player, similar to how strongholds and helltides work. After they defeat all of the waves, the Tomb Lord rises again, but this time, he has an army with him, comprised Skeletons, Zombies, Ghosts, Vampires, and the Drowned. After the player defeats all of these, they are granted the Unique Staff - Skull Collector.
- +20.0% Damage against Vampires, Skeletons, Zombies, Ghosts, and Drowned [20.0]%
- +2 Mana On Kill [2]
- +20.0% Maximum Mana [20.0]%
- Lucky Hit: Up to a 20% Chance that the next Vampire, Skeleton, Zombie, Ghost, or Drowned you slay will join your side for the next 20 seconds.
https://imgur.com/e9ayoXR
Example 2:
For the second example, we have Chance Guards, Unique Gloves. Player would, as in the first case, first have to find the recipe, then the base item (in this case gloves) and use the forge NPC to start the quest. This time, however, the task is different, and is not very reliant on the difficulty of the content. Since we are crafting the item based on improving our luck, the task at hand revolves around luck:
- Collect the lucky coin from Avarice, the Gold Cursed: Open the Avarice, the Gold Cursed treasure chest to find the coin.
- Collect treasure realm portal residue: Let the treasure goblin open a portal to the treasure realm, then kill it.
Additionally, this recipe requires spending 1.000.000 gold.
- +150 Armor [150]
- +10.0% Attack Speed [10.0]%
- +22.0% Lucky Hit Chance [22.0]%
- Lucky Hit: Up to a 15% chance that the next monster you hit will drop between 1 and 26,666 gold.
https://imgur.com/zTfkOu3
Example 3:
For the third example, we have String of Ears, Unique Pants. The side quest in this case could be:
- Vanquish foes in battle and collect your trophies: Collect 10 ears of different worthy opponents in battle.
Here the item could scale of the ears you used to craft it. It would also give you a mobile gallery of ears that you can show off when you are inspected. If you collect 10 ears of 10 different level 100 players, you will have the perfect string of ears (shown below).
- While Injured, Your Potion Also Grants 20% Maximum Life as Barrier
- 28.0% Damage to Elites [14.0 - 28] %
- 3 Potion Capacity [2 - 3]
- 7.3% Damage Reduction [3.1 - 7.3] %
- 2.0% Life Steal [1.0 - 2.0] %
- 24.5% Magic Resistance [12.2 - 24.5] %
https://imgur.com/sohVBcj
10 ears of level 50 players yield minimum rolls, and the scale is all in between. Since there 5 properties to this item, and 10 ears required, when crafting, perhaps a pair can be combined at random from all 10 to create each property. For example ears of level 56 and 90 players would average out to level 73. If these two would roll on Damage to Elites, they would give a result of 20.4% [ (73-50) * 2 / 100 * (28%-14%) + 14% ]
Crafting Rare from Magic
In the major towns, a new service is available at the blacksmith’s - Improve item. It takes a blue item base, 6 gems, and a potion/incense. Flavorwise, the gems are used to improve the steel, or make the item conduct magical energies better; and the potion/incense is added to the oil for tempering. Crafting here is very deterministic. If you find a good blue item base, you select the two new properties by choosing which gems to add and which potions. In the following example, we will use the blue focus.
- +5.0% Critical Strike Chance [1.8 - 5.0] %
- +10.0% Cooldown Reduction [4.4 - 10.0]%
- (required level 70)
It rolled perfectly on Crit chance, and Cooldown reduction. We want to make it more perfect, by adding some crit damage and attack speed, so we will go with 6 Royal Emeralds, and the Heady potion of Assault. Notice that the potion’s level requirement is 95, so the crafted item will inherit this.
- +5.0% Critical Strike Chance [1.8 - 5.0] %
- +10.0% Cooldown Reduction [4.4 - 10.0] %
- +14.0% Critical Strike Damage to Vulnerable Enemies [10.0 - 15.0] %
- +3.5% Attack Speed [3.5 - 7.5] %
- (required level 95)
https://imgur.com/7CI9Vz2
Oh, bad luck. We have rolled only 3.5% on Attack speed, BUT, this property would not be normally found in a focus. The range from which these stats are pulled is always at maximum tier 4 (out of 5), so the minimum and maximum values you can get are always lower than what you can find on gear regularly. The upside, as I mentioned before, is that you can have stats that do not normally appear on gear. Also, crafted gear should be a stepping stone towards better gear you can find from monsters, OR enable some fringe or fun builds.
Here’s another example. Now we are using Royal Skulls and the Chorus of War potion. The pot has a required level of 100, so our crafted item will be level 100.
- +5.0% Critical Strike Chance [1.8 - 5.0] %
- +10.0% Cooldown Reduction [4.4 - 10.0] %
- 22 Life On Kill [10 - 25]
- 1 Rank of War Cry [1]
- (required level 100)
https://imgur.com/g7ip2jo
Casting another class skill spends your class’s resource in the equal amount as it would in this case a Barbarian.
What happens if you take lower level potions and gems? Well, you get lower value ranges for the stats you want. For example, let’s say we spent 6 Crude Rubies instead of Royal, and used a Weak Acrobatics Elixir. Rubies would roll on a table for item power 0 to 149, as would the potion resulting in the following item.
- +5.0% Critical Strike Chance [1.8 - 5.0] %
- +10.0% Cooldown Reduction [4.4 - 10.0] %
- +4.0% Overpower Damage [1.5 - 6.0] %
- +1.0% Dodge Chance [1.0] %
- (required level 70)
https://imgur.com/QFax99i
The level requirement has not changed, since both the potion and the gems were very low level.
What happens if both the gems and the potion would add the same stat or if the stat has already been rolled on the base item? Answer, you cannot craft the same stats on an item that already exists or that would clash with one another. Reason is to prevent items from getting ridiculous crit chances, or damage, etc.
Can you reroll stats that you crafted on an item? Answer, you cannot, since most of these stats are not “native” to the items they are grafted on.
Does improving the item quality affect the stats? Answer, yes, they would grow like other stats.