I did my best to search the "most common suggestions" list, but didn't find anything like this. So...
I think it would be really nice to have a dropdown menu on the bottom of the backpack, to quickly view all available backpack-craftable items. Scrolling from side to side, especially once you've unlocked all of the backpack "recipes", becomes very tedious.
Another function that could work well with the backpack is the ability to drop an ingredient (say, resin) into the bottom-left slot (even if it was currently set to zinc or something), and the output would automatically switch to one of the resin recipes, and you could then hit the left or right arrow to go to the next resin recipe. [Not sure if I got my message across effectively here, but I did my best]