Introduction
It's common opinion that bundles could use some improvement. While they're definitely great to have, obtaining them is tedious and using them cumbersome. Lots of suggestions have been made to improve the bundle: an inventory you can directly interact with, the ability to craft bundles with leather, etc. However, I think I have something new to bring to the table that would also help make bundles more usable.
How It Works
By default, bundles are tied. Tied bundles act like they currently do, but they'll always have a full bundle sprite. Nothing spectacular here.
By double-clicking a bundle, you can untie it. Any item that the player picks up can go into an untied bundle IF that same item is already in the bundle. If you put a dandelion and a diamond in an untied bundle, all dandelions and diamonds the player picks up will go into that bundle. Untied bundles have an open bundle sprite. Double-click the bundle again to tie it.
To indicate this behavior to the player, bundles could have a tooltip like "Double-click to (un)tie".
How It's Useful
Untied bundles can be immensely useful while mining, especially pre-endgame. Materials you get in low amounts (e.g., raw gold, diamonds, granite, flint) can be put into an untied bundle or two. Then any time you pick up those items, they'll be redirected to a bundle. This makes underground inventory management way easier. If you mine with bundles, you no longer have to continuously put stuff into bundles; simply insert one of each item you want in bundles and go about your mining trip.
They're also helpful during nighttime expeditions. Any somewhat common but potentially useful drops (e.g., string, gunpowder, bones) can be put into untied bundles. Then all of those basic mob drops can be automatically allocated to bundles!