Okay so hear me out.
Vikings were pretty well-known for the use of hallucinogenic mushrooms and other substances for a multitude of reasons, not least of which was the mind-altering effect that supposedly granted their spiritual leaders visions and the ability to speak with their gods. Dreams are already present in the game in the form of tasteful immersion, but otherwise doesn't serve much purpose. Mushrooms are already present in two variations as consumables. Hell, even brewing is present.
My idea is that we're able to either find or purchase a special mushroom or herb that we can either eat directly, or brew into a kind of drink. Once consumed, the next time you fall asleep your dream points you to a new place on the map that holds some special importance – could be a treasure, a special mob or fight, a place you haven't yet explored, or a hot air balloon that takes you to the magical tree in the sky. This could give even more incentive to sail and explore the map without forcing anyone into it.
I'm not saying that dreams necessarily need to have any mechanical importance, but I feel like all the building blocks are already there and it could be a pretty seamless addition.