Let me state upfront my current gaming philosophy so you know where my thoughts on crafting are coming from:
"The object of this game is to advance your first level character into as much of a superperson as you can."
I would like crafting rules to support that objective, by enabling PCs to convert resources that they do not want (e.g. gold) into resources that they want (e.g. magic items), effectively bringing back magic item wish lists and magic item shops.
I had previously done an analysis of a "typical" distribution of magic items across a 1st to 20th level campaign and obtained the following guidelines on magic items per character which I use when planning my own adventures and campaigns:
1 common consumable every level from 1 to 5.
1 uncommon consumable every level from 6 to 10.
1 rare consumable every level from 11 to 15.
1 very rare consumable every level from 16 to 19.
1 legendary consumable at level 20.
1 uncommon permanent item at level 4, and another at level 7.
1 rare permanent item at level 10 and another at level 13.
1 very rare permanent item at level 16.
1 legendary permanent item at level 19.
A separate analysis of "typical" monetary treasure (which I may have posted, but redid recently) resulted in the following guidelines:
650 gp total from levels 1 to 4.
20,400 gp total from levels 5 to 10.
109,000 gp total from levels 11 to 16.
507,000 gp total from levels 17 to 20.
If we adopt the rough convention that an uncommon magic item is worth 500 gp, a rare magic item is worth 5,000 gp, a very rare magic item is worth 50,000 gp, and a legendary magic item is worth 250,000 gp, and assuming a magic item shop or crafting rules which enable the conversion of gold to magic items, this translates into an additional uncommon magic item at level 4, four additional rare magic items between levels 5 to 10, two additional very rare magic items between levels 11 to 16, and two additional legendary magic items between levels 17 to 20. (As 250,000 gp for a legendary magic item seems an outlier in terms of the progression, maybe the value should be changed to 500,000 gp, in which case each PC would only get one additional legendary item.)
With that in mind, I'd like to see crafting rules in line with the following:
1. Crafting takes place during downtime
This is a personal preference when it comes to pacing campaigns as I'm not a fan of the PCs rushing from adventure to adventure. Crafting during downtime gives the PCs a reason to remain in their base of operations in order to get better prepared for the next adventure - and to spend gold, of course.
2. Crafting should take place in a reasonable amount of time
That said, I'm not keen on high-level PCs spending 250 days to craft a legendary magic item. I'm thinking 10 days of work should be enough, and the types of items a PC can craft should be based on level: common and uncommon at levels 1 to 4, rare at levels 5 to 10, very rare at levels 11 to 16, and legendary at level 17 to 20.
3. Special components are helpful but not necessary
Gold is enough of a gate. You don't need the PCs to go on special quests to find exotic components as well. That said, a helpful DM could choose to place items in the adventure that either count towards the gp requirement or reduce the crafting time required.