A more complex form of Crafting that allows chance of Failure, Critical Failure, and Critical Success, as well as time being determined by a number of rolls instead of set. I do have a concern about the number of rolls needed for Crafting more powerful items, but as this is for downtime activities, it doesn't need to happen at the table. I'll provide a few example afterwards, the longest of which took about 20 minutes to complete. Feel free to give comments or suggestions as this is still being playtested.

CRAFTING

PART I: The Idea

The first part of any invention is the idea. Describe what the item is, and what it does.

PART II: Formula

Before any project can be started, it needs a formula. Formula can mean Alchemical Formula, Design Schematics, Blueprints. In this portion, figure out what spells will be needed to make the magic happen. This will adjust the DC needed for an Int (Arcana). The DC starts at 10 and add 1 for each Spell level enchanted into the item. Make 1 check per day. A single success is all that is needed to develop the necessary formula. A failure will increase the DC by 5 for the next day's check. The DM may rule that eventually this is just beyond the character's ability. If the character has access to an existing item, this roll is made with Advantage.

An item's rarity is determined by it's DC: 1-10 Common, 11-14 Uncommon, 15-19 Rare, 20-24 Very Rare, 25+ Legendary.

PART III: Materials

Now that the formula is created, it's time to start gathering materials for the magic item. There are 4 types of materials needed for each item: Vessel, Spell Components, Spell Costs, and Special Ingredients. The cost for each of these is determined by the Item's Rarity rating. The more powerful an item is, the more expensive it is to make.

Vessel: The vessel is the form that the item takes, whether it be sword, bag, belt, or armor. The base cost is that of a normal item of that type.
    Common: (Cost/2)
    Uncommon: (Cost)
    Rare: (Cost*10)
    Very Rare: (Cost*100)
    Legendary: (Cost*1,000)

Spell Components: This is fairly self-explanatory. Any cost for spell components are added into the cost of the item.

Spell Costs: Enchanting an item to take a particular spell has a cost. Every level of spell cast into an item has a cost based on the Items' Rarity rating.
    Common: 10 gp/spell level
    Uncommon: 50 gp/spell level
    Rare: 100 gp/spell level
    Very Rare: 500 gp/spell level
    Legendary: 1,000 gp/spell level

Special Items: Magic items need a catalyst in order to manifest the magical energies. The number of special items is based on the rarity of the item. This can be a rare metal, a special plant, the heart of a creature, etc. What is actually needed is determined by the DM and a special quest may be necessary to retrieve it. For every special item, remove one spell level from the cost of the item.
    Common: 0 Special Items
    Uncommon: 1 Special Item
    Rare: 2 Special Items
    Very Rare: 3 Special Items
    Legendary: 4 Special Items
If the only enchantment is adding a modifier to the item (a +1 longsword for example), remove 1 necessary Special Item.

PART IV: Workshop

Now that the materials have been acquired, a place to work on the item needs to secured. For nonmagical items, a simple workshop can be used. For a magical item, however, a magical workshop is needed. The more powerful the item is, the more well-equipped the workshop needs to be. Magical workshops also have daily maintenance costs which include upkeep, salaries, common materials, etc. When not in use, the cost is halved. Each level of workshop has a maximum number of hirelings who can work at a time.

Additionally, the more well-equipped the workshop is, the more tool-types can be used. A simple forge is not equipped to use wood-working tools, for example, but a better-equipped workshop could have space for up to 5 tool-sets. A workshop can be upgraded. A lower-quality workshop can be upgraded to a higher-quality one by paying for the difference between the two.

Common: Construction Cost: 500 gp; Construction Times: 10 days; Maintenance Cost: 2 gp/day; Max Hirelings: 1; Max Toolsets: 1
Uncommon: Construction Cost: 1,000 gp; Construction Times: 30 days; Maintenance Cost: 5 gp/day; Max Hirelings: 1; Max Toolsets: 2
Rare: Construction Cost: 5,000 gp; Construction Times: 60 days; Maintenance Cost: 10 gp/day; Max Hirelings: 2; Max Toolsets: 3
Very Rare: Construction Cost: 10,000 gp; Construction Times: 100 days; Maintenance Cost: 25 gp/day; Max Hirelings: 3; Max Toolsets: 4
Legendary: Construction Cost: 25,000 gp; Construction Times: 150 days; Maintenance Cost: 50 gp/day; Max Hirelings: 4; Max Toolsets: 5

If a character uses a higher quality Workshop to make a lower rarity Item, it still has the Maintenance Cost for the higher quality workshop. However, the maximum number of Hirelings are equal to the workshop, not the item. So if a player is making an Uncommon item in a Very Rare level workshop, they can use up to 3 hirelings, but the Maintenance cost will be 25 gp/day.

PART V: Crafting the Item

The formula has been developed, the materials gathered, the workshop secured. Now is the time to start the long process of creating a magical item. The amount of time it takes is based off the total cost of the item. Every day the character is working on it, they make an Int (Tool Set) ability check based on the DC of the formula. This is based on 8 hours working on the item. A character can choose to to push, working 16 hours and getting 2 rolls per day, but the second roll is at Disadvantage.

When crafting the item, a person who can cast the appropriate spell must be on-hand. A scroll can be used as an alternative, but the chances of failure do apply. The cost to hire an NPC is (LevelxLevel)x10 + (Material Component x 2) + 10%.

On a success, the character pays (5 gp * the amount succeed by(minimum of 1) * the number of hirelings(if applicable)). The first hireling gives Advantage on this roll, but a hireling is not required. When the total cost is reduced to 0, the item has been built. There is also a 5% non-cumulative chance of a breakthrough which doubles the amount spent for the day.

On a failure, the character pays 5 gp , but it does not reduce the total cost. There is also a 5% cumulative chance per failure that a critical failure occurs. Once a critical failure occurs, the chance resets to 5%.

PART VI: Critical Failure

Every creation has a chance of something going wrong. A simple failure simply costs money and time. A critical failure can have disastrous results. Every failure increase the chance of critical failure by 5% (1 failure 5%, 2 failures 10%, etc). If a critical failure occurs, roll d10 on the chart below to see the results.

Die Roll    Result
1-3    A flaw occurs, which can be repaired. The next day can be spent fixing the problem, gold being spent but no progress is made. If the flaw is not fixed, there is a 5% chance every time it is used to stop functioning.
4-5    A flaw occurs, which cannot be repaired. There is a 5% chance every time it is used to stop functioning.
6-7    The item is ruined. All progress is lost. Any material value already used must be replaced
8    The item is ruined. All progress is lost and the materials are corrupted. All materials must be replaced.
9    Magic is dangerous. The item explodes, causing (1d10 * Spell levels) damage, halved on a Dex Save with DC of Item. The item is ruined. All progress is lost and the materials are corrupted. All materials must be replaced.
10    The item is cursed. The DM determines the nature of the curse.

PART VII: Breakthrough (Optional)

Instead of doubling the progress, the DM may determine that a special perk occurs. Roll d10 on the chart below for the results.

Die Roll    Result
1    Compass: The bearer can use the item to determine north.
2    Delver/Altimeter: The bearer can use the item to determine how far below or above sea level they are.
3    Bane: The item provides Advantage against one particular type of creature. (determined by DM)
4    Lightweight: The item weighs half as much as normal.
5    Mundane: The item does not give off a magical aura.
6    Divine: The item can act as a Holy Symbol.
7    Focus: The item can act as a Spell Casting Focus.
8    Healer: The item heals 1d4 points of damage per turn as a Bonus Action to the bearer.
9    Detector: The item glows when near one particular type of creature. (determined by DM)
10    Spell Saver: The item can store 1 spell level

Example 1: Longsword +1

Character:    Sirath Leinol: Level 10 Artificer. Int 16. Arcana +7. Tool Use(Int): +11

Idea:    A simple +1 Longsword. They have a version already and want to copy it.
Formula:
    +1 Longsword:     DC 10
    +1 Longsword -  Magic Weapon (2nd level)    DC +2
    Total        DC 12 (Uncommon)
    Int (Arcana) with Advantage    10 failure
    Revised DC:    17
    Int (Arcana) with Advantage    14 failure
    Revised DC:    22
    Int (Arcana) with Advantage    23 success
    Time spent    3 Days
Materials:
    Vessel: Longsword    15 gp
    Spell Cost: Magic Weapon (2nd Level)    100 gp
    Spell Components: None    0 gp
    Special Materials: None (Uncommon 1 - 1 for  Modifier Only)    0 gp
    Total Cost:    115 gp
Workshop:    Uncommon Minimum. 1 Hireling. Blacksmith Tools    5 gp/day
Crafting:
    Day 1: 12 success (67% on breakthrough)    - 5 gp (110 gp)
    Day 2: 19 success (73% on breakthrough)    -35 gp (75 gp)
    Day 3: 12 success (61% on breakthrough)    -5 gp (70 gp)
    Day 4: 20 success (77% on breakthrough)    -40 gp (30 gp)
    Day 5: 14 success (82% on breakthrough)    -10 gp (20 gp)
    Day 6: 25 success (25% on breakthrough)    - 20 gp (0 gp)
    Time Spent: 6 days    30 gp    
    Total Cost:    145 gp

Example 2: Quarterstaff of Storms

Character:    Sirath Leinol: Level 10 Artificer. Int 16. Arcana +7. Tool Use(Int): +11

Idea:     A magical quarterstaff that can do lightning or thunder damage.
Formula:
    Quarterstaff of Storms:     DC 10
    +1 Quarterstaff -  Magic Weapon (2nd level)    DC +2
    1d6 Thunder Damage - Arcane Weapon (1st level)    DC +1
    1d6 Lightning Damage - Arcane Weapon (1st level)    DC +1
    Total        DC 14 (Uncommon)
    Int (Arcana)    23 success
    Time spent    1 Day
Materials:
    Vessel: Quarterstaff    2 sp
    Spell Cost: Magic Weapon (2nd level)     100 gp.
    Spell Cost: Arcane Weapon (1st level)    50 gp
    Spell Cost: Arcane Weapon (1st level)    50 gp.
    Spell Components: None    0 gp
    Special Materials: 1 of 1    -50 gp
        (Heartwood from a Lightning Struck Tree)
    Total Cost:    150  gp 2 sp
Workshop:
    Uncommon Minimum, 1 Hireling, Woodcrafting Tools    5 gp/day
Crafting:
    Day 1: 13 failure (34% on catastrophic failure(5%))    - 5 gp No Progress (150 gp 2 sp)
    Day 2: 23 success (70% on breakthrough)    -45 gp (105 gp 2 sp)
    Day 3: 18 success (90% on breakthrough)    -20 gp (85 gp 2 sp)
    Day 4: 31 success (11% on breakthrough)    -85 gp (0 gp 2 sp)
    Day 5: 24 success (82% on breakthrough)    -2 sp (0 gp)
    Time Spent: 5 days    25 gp    
    Total Cost:    180 gp, 2 sp

Example 3: Bead of Force

Character:    Sirath Leinol: Level 10 Artificer. Int 16. Arcana +7. Tool Use(Int): +11

Idea:     Has heard of the Bead of Force but does not have one
Formula:
    Bead of Force:     DC 10
    Residual Sphere (4th level)    DC +4
    4d6 Force Damage - Magic Missile (2nd level)    DC +2
    Total        DC 16 (Rare)
    Int (Arcana)    14 failure
    Adjusted DC    DC 21
    Int (Arcana)    12 failure
    Adjusted DC    DC 26
    Int (Arcana)    18 failure
    Adjusted DC    DC 31 (max roll they could make is 27)
    Time spent    3 Days (ruled beyond capability)

Example 4: Censer of Controlling Air Elemental

Character:    Sirath Leinol: Level 10 Artificer. Int 16. Arcana +7. Tool Use(Int): +11

Idea:     While incense is burning in this censer, you can use an action to speak the censer's command word and summon an air elemental, as if you had cast the conjure elemental spell. The censer can't be used this way again until the next dawn.
Formula:
    Censer of Controlling Air Elemental:     DC 10
    Conjure Elemental (5th level)    DC +5
    Total        DC 15 (Rare)
    Int (Arcana)    9 failure
    Adjusted DC    DC 20
    Int (Arcana)    26 success
    Time spent    2 Days
Materials:
    Vessel: Censer (DM Determined Price 200 gp)    2,000 gp
    Spell Cost: Conjure Elemental (5th level)     500 gp.
    Spell Components: None    0 gp
    Special Materials: 1 of 2     -100 gp
        (Incense made from a plant which survived a hurricane)
    Special Materials: 2 of 2     -100 gp
        (Bronze exposed to the elemental plane of Air)
    Total Cost:    2,300 gp
Workshop:
    Rare Minimum, 2 Hirelings, Tinker's Tools    10 gp/day
Crafting:
    Day 1: 19 success (38% on breakthrough)    - 40 gp (2,260 gp)
    Day 2: 22 success (35% on breakthrough)    - 70 gp (2,190 gp)
    Day 3: 20 success (99% on breakthrough)    - 50 gp (2,140 gp)
    Day 4: 13 failure (46 % on critical failure(5%))    - 5 gp No Progress (2,140 gp)
    Day 5: 18 success (91% on breakthrough)    - 30 gp (2,110 gp)
    Day 6: 12 failure (13% on critical failure(10%))    - 5 gp No Progress (2,110 gp)
    Day 7: 17 success (91% on breakthrough)    - 20 gp (2,090 gp)
    Day 8: 23 success (55% on breakthrough)    - 80 gp (2,010 gp)
    Day 9: 22 success (52 % on breakthrough)    - 70 gp (1,940 gp)
    Day 10: 14 failure (37% on critical failure(15%))    - 5 gp No Progress (1,940 gp)
    Day 11: 27 success (55% on breakthrough)    - 120 gp (1,820 gp)
    Day 12: 27 success (50% on breakthrough)    - 120 gp (1,700 gp)
    Day 13: 25 success (97% on breakthrough)    - 100 gp (1,600 gp)
    Day 14: 29 success (31% on breakthrough)    - 140 gp (1,460 gp)
    Day 15: 27 success (8% on breakthrough)    - 120 gp (1,340 gp)
    Day 16: 21 success (99% on breakthrough)    - 60 gp (1,280 gp)
    Day 17: 15 success (45% on breakthrough)    - 10 gp (1,270 gp)
    Day 18: 14 failure (100% on critical failure(20%))    - 5 gp No Progress (1,270 gp)
    Day 19: 14 failure (8% on critical failure (25%))    - 5 gp No Progress (1,270 gp)
        Critical Failure: A flaw occurs, which cannot be repaired.
        There is a 5% chance every time it is used to stop functioning.
    Day 20: 19 success (90% on breakthrough)    - 40 gp (1,230 gp)
    Day 21: 15 success (38% on breakthrough)    - 10 gp (1,220 gp)
    Day 22: 12 failure (33% on critical failure (5%))    - 5 gp No Progress (1,220 gp)
    Day 23: 24 success (33% on breakthrough)    - 90 gp (1,130 gp)
    Day 24: 12 failure (65% on critical failure (10%))    - 5 gp No Progress (1,130 gp)
    Day 25: 16 success (93% on breakthrough)    - 10 gp (1,120 gp)
    Day 26: 21 success (10% on breakthrough)    - 60 gp (1,060 gp)
    Day 27: 28 success (23% on breakthrough)    - 130 gp (930 gp)
    Day 28: 26 success (58% on breakthrough)    - 110 gp (820 gp)
    Day 29: 14 failure (55% on critical failure (15%))    - 5 gp No Progress (820 gp)
    Day 30: 20 success (50% on breakthrough)    - 50 gp (770 gp)
    Day 31: 18 success (69% on breakthrough)    - 30 gp (740 gp)
    Day 32: 19 success (93% on breakthrough)    - 40 gp (700 gp)
    Day 33: 30 success (71% on breakthrough)    - 150 gp (550 gp)
    Day 34: 17 success (42% on breakthrough)    - 20 gp (530 gp)
    Day 35: 25 success (25% on breakthrough)    - 100 gp (430 gp)
    Day 36: 25 success (72% on breakthrough)    - 100 gp (330 gp)
    Day 37: 27 success (20% on breakthrough)    - 120 gp (210 gp)
    Day 38: 14 failure (76% on critical failure (20%))    - 5 gp No Progress (210 gp)
    Day 39: 13 failure (69% on critical failure (25%))    - 5 gp No Progress (210 gp)
    Day 40: 22 success (63% on breakthrough)    - 70 gp (140 gp)
    Day 41: 29 success (71% on breakthrough)    - 140 gp (0 gp)
    Time Spent: 41 days    410 gp
    Special Hireling: 9th level Wizard to cast Conjure Elemental    891 gp
    Total Cost    3,651 gp