I'm going to say Block Swap. I often spend HOURS laying out the entire foundation of my large base with dirt blocks, but then have to spend large amounts of time removing that dirt and replacing it with the desired material. The ability to swap out chests will be SO USEFUL as well, because I like to have a singular gigantic chest area in the middle of the base, and each chest is a different chest material AND paint job to differentiate them. I don't just give them a name. The "at a glance" factor is really important, and being able to swap chests out for other kinds of chests will be absurdly useful. I'm just so massively grateful for it. Quality of Life changes are what I'm all about.
Such as?