For example, the Turtle Shell backpack could give you +1 armor toughness at all times, the Piglin Pack could make you neutral to Piglins, etc.

A big turnoff to this idea is that the only way for this to be balanced would mean you have to lock in your customization option, such that you can't just turn on the Piglin Pack in the Nether and then back to the Turtle Shell when you get home, but rather, you need to own two separate backpacks.

I made a paste with property ideas for each unlockable backpack. Feel free to ignore these entirely if the feature itself is simply not interesting to you.

https://pastebin.com/raw/BSxBknss

Unfortunately this doesn't play well with the default settings. Backpacks can't be removed if there are items contained inside, so two backpacks is not an option. I think the idea of bonus is good though, maybe just a very small bonus. I'll keep this open, it's unlikely any content updates will be coming to this mod anytime soon since I am working on other projects.