Current candidates for crop item sources:

• Planted by farms on villages (I like this)
• Dropped from grass (I'm leaning against it, as it causes inventory clutter and excludes onions)
• Zombie drop (maybe?)
• Chest loot, where carrots and potatoes usually appear

A more advanced idea to consider: Crop Patches that spawn wild in certain biomes or habitats, similar to Pumpkins and Melons:

• Cabbage Patches can be found on the shores of Beach biomes;
• Tomato Patches can be found in arid intersections, such as Savannas;
• Onion Patches can be found amidst Allium patches in flower fields or plains.

I managed to get Cabbage, Tomato and Onion to be generated in loot for the following chests:

• shipwreck_supply
• village_biome_house

The current method adds pools via the LootTableLoadEvent, which may not be the ideal way to implement this. I will need to study GlobalLootModifiers, especially to add conditions for things such as the Knives.