deep dark sea

I think it would be very cool to have parts of the ocean lead down very far, almost down to bedrock. 

A deep sea biome would barely have any light reach it, it would be pitch black making it difficult to explore without good equipment such as sealanterns. you would graddualy take damage from the waterpressure if not wearing good armour as to make it more difficult to access in the early game.

It would have very good loot in it, whether it be exposed ore or ruin chests. It might contain stuff like diamonds or diamond gear, enchanted books, golden apples or other rare resources. 

Plenty of dangerous mobs would inhabit the area making it difficult to explore, drowneds, guardians and maybe more new deepsea creatures would all patrol the place making sure survival is difficult. 

Glowsquids could spawn around the ruins to guide players to them, giving them an acctual use. 

I think this would be a great biome to have, especially since the ocean terrifies a lot of people making it an exiting experience to explore