I've had this idea for a while. I don't think I'll be able to make it myself until I'm a LOT more familiar how these things are done, but I might as well get the idea out there.
so anyway, I don't really like trading with villagers. their UI is kind of awkward, and generally they have garbage trades anyway...
so like, what if there were a specific villager type who was a shopkeeper, and he had a totally different UI, and instead of offering individual trades, he had a whole shop screen? like it shows all the items at once, and lets you sell stuff to him and whatever?
I drew up a fake version of the UI I had in mind as an example.
so basically I envision it like this. you can click on, (or drag) items from his inventory to the first slot. the emeralds next to that show the price of the item, and you drag emeralds from your inventory to the second slot to pay for the item.
I could also see it working as a way to sell certain items too. most items like dirt and stone he wouldn't buy, or at least wouldn't offer emeralds for, but stuff like armor, or iron ingots or other things that villagers normally buy you could drag from your inventory into the first slot. he would then make an offer (where the price is shown normally) and then you could take the emeralds he offered in exchange for them.
it sounds kind of complicated to me, but honestly I don't know much about modding at the moment.... let me know what you think though.