This is a simple one and I’m sure it has been mentioned in the past but having more wildlife in the ocean would be amazing. Dolphins, whales, different kinds of sharks (maybe a big great white?), bigger fish in deeper regions (i.e. - tuna) and/or giant schools of fish.
Sea of Thieves has one of the best environments in the industry, and I genuinely feel like more diverse wildlife to fill up the ocean would make the experience of roaming the seas so much better. Imagine dolphins swimming in front of the ship or a massive whale breaching the surface of the ocean in the distance! So much potential!