Seeing as how Villager trading is going to be updated as confirmed by Agnes (to what lengths, I don't know- but I'm excited!), it would be a beautiful addition that their trades become data-driven, modifiable through datapacks!
I adore the datapack functionality and currently, I heavily and lightly tune advancements, loot tables, functions, and recipes.
Adding support for trading would prove a solid feature to me :D
There has been trading support for a while now.
Putting the trades in a JSON file instead of modifying through functions would be great. It would be great if we could add trades that are tied to a certain biome. Like purchase sweet berries only from snowy taiga villagers.
As someone who plays in peaceful frequently, it would be nice to have trades that are only available in peaceful because they would break a survival game. Things like blaze powder.