The title of this issue is vague because otherwise it'd be huge, haha

While working on my mod VanitySlots, I noticed a few things I had to find workarounds for:

1. There doesn't seem to be an easy way to let already existing items enter slots. What if you want to have a slot that can have whatever in it? For example, if I want to be able to put slimeballs in a slot, I would have to write a mixin. This isn't too bad, but for stuff like armor...
2. There should be a way to make a new slot act like vanilla slots. Without this, you have to write a mixin for things like ArmorItem.
3. There should be some sort of way to choose whether shift-clicking an item goes into the vanilla slot first instead of the new slot. For example, if you add a slot in the chest group, and you shift-click, and the item can go in either the custom slot or the vanilla chestplate slot, there should be a way to make it go into the vanilla chestplate slot if it's empty, instead of going into the custom slot first.

All of these things actually exist (except part of 3).
When you define a slot you can define how it chooses to validate items, see the Trinkets chest:cape slot and how it allows elytra: https://github.com/emilyalexandra/trinkets/blob/master/src/main/java/dev/emi/trinkets/TrinketsMain.java#L23
Slots also have a property called disableQuickMove which is public and you can modify to disable shift clicking into that slot.
The only thing Trinkets does not have is more precise control over which items can and can't be shift clicked into slots, but that's a low priority issue as I'm currently rewriting trinkets for its 3.0.0 version which will include a lot of changes by itself

Alright, thank you!