This might be more suited for a potential 1.13 update, but I would really love to see one of my favorite races in the Mythos- the Deep Ones- make an appearance in some form. Perhaps they could spawn in a beach biome-specific variant of villages, alongside villager-hybrids, similar to Innsmouth, or near ocean monuments. Maybe they could drop an item that gives you unlimited water breathing but gives you slowness/weakness (increasing severity) and eventually suffocation out of water, or lets you command guardians/drowned but scares away turtles, or makes you swim faster but causes dolphins to attack you.