Won't be implemented until after 1.0.0. Not even 100% sure on how to implement it down to a T, but would still be freaking cool.

Attaching the amount of the item being given is shown, there's a few issues with this:

1. How do we show the amount of emeralds each item needs?
2. How do we show secondary item costs (such as the gravel + emerald = flint trade)?

One way to do this would be holding an emerald would show the items the villager is selling along their arms, and holding an empty hand would have them hold the items they're buying for emeralds.

One way these issues could be solved is the user points at/selects the item he wants to trade, and then these details are shown. In the end, this is still a pretty GUI solution, but I don't think there is a GUI-less solution considering how complex the Villager system is (unless we're planning on supporting pre 1.14 :P /s)

A swiping motion would be a good way to cycle between trades.

Getting villager to look at you and hold its output based on what you hold. Can swipe (or also use arrows as an alternate option) to change between trades.