Datapacks

Almost everything about how traps behave is data, so a datapack can change it without touching code.

What	Where	Details
Which items are bait, and how good they are	data/<namespace>/bait/*.json	Bait files
Which items are nets, and what they do	data/<namespace>/net/*.json	Net files
What each biome catches	data/fishtrap/loot_table/fish_trap/*.json	Catch tables
Which biomes count as warm ocean, cold ocean or swamp	data/fishtrap/tags/worldgen/biome/fish_trap/*.json	Catch tables
Advancements for trap catches	data/<namespace>/advancement/*.json	Advancement trigger

All of it reloads with /reload. Players' tooltips and JEI update straight away.