Add option to sort Alphabetically, Rarity, Item Count, Item Tags or Item Type.

Supported out of the box:

• Alphabetically - set "Mod Settings -> Sort Order" to Item Name

Supported with custom rules see docs and some more docs

• Tags
• Item Type

Minecraft has no real concept for:

• Rarity

How about item count? By sorting the items by how much of it is present in the inventory.
It would be possible to add rarity by having items names and item tags in a dictionary with keys as tier and values as a list of items sorted by how easy the item is to obtain and how much playtime you need to get that item. Similar on how a crafting table as starting tier, an enchanting table as mid tier, and dragon egg/elytra as end game tier

The problem with Rarity is that it's relative and depends on external factors

• other mods - adding new items will change how Rarity is evaluated in unpredictable ways
• player experience
• player preferences
• player view on what is hard for them to do

So I guess the custom rules are the right way to support this too.

Stacks and Slots

IPN works on stack level so to sort by item count with the current sort engine it would mean to ignore the items and just compare the stack sizes. This will lead to random arrangement of types. I'm sure you don't want this.

I will check how feasible is to implement something like this as a Post action (see Gui Settings -> Post Action)