Mermaid Helmet- Loses the ability to breed if only one is given. Allows breathing underwater and pushes drowned away drowned when hit. Villagers equipped in it will prefer waterlogged beds and doors and won't float on water, like skeletons. Giving them a second one will allow them to initiate breeding underwater (which is given to the baby villager upon creation).
Construction-Turtle Helmet and Conduit. Must be enchanted with Water Breathing before being given to a villager, and can only be enchanted with Water Breathing.
Hoglin Helmet- Hoglins now attack villagers without it. When it's equipped, Hoglins won't attack them. Makes them immune to fire damage, and if near a nether portal in the overworld, they will attempt to reach it.
Construction -Gold Helmet, Netherite Scraps, and Magma Cream/Blaze Powder/Blaze rod (Not Shure which would be best). Must be enchanted with Fire Protection before being given to a villager, and can only be enchanted with Fire Protection.
Ender Helmet- If they fall enough meters that they'd take fall damage, they'll instantly teleport to a nearby block. Occasionally teleports, similar to endermen, but will prefer to teleport inside villages.
Construction- Netherite Helmet, Eye of Ender (Maybe also some End item). Must be enchanted with Feather Falling to be given to a villager, and can only enchanted with Feather Falling.
I think this way, it'll open a way to make villagers better suited to other environments without changing the initial behavior of villagers, and so I can make an actual underwater village.