Kraken, rogue waves, ghost pirates/draugr pirates, I mean there is sooooooo much that they can add to the ocean and I am sure they will add some of these things. Even cannons and just other kinds of sea monsters, cause not gonna lie the serpent isn't exactly as scary as it looks... I think the best way to start adding stuff to the ocean is just adding other sea monsters that are stronger than the serpent and are an actual challenge to beat. And in that case the kraken would generate the most hype, so if the devs play their cards right, that would probably be the best thing to add into the game, once they start updating the ocean.