I'm a fan of the Kraken and Megalodon fights in Sea of Thieves, but I wish there were more ocean giants you could encounter on the ocean. I think it'd be neat if there was some kind of sea serpent boss you could encounter in the Devil's Roar as either a world event or maybe a random encounter, would seem like a fitting location. It could have a nesting ground that if you sail to close it gets aggravated and the fight begins, but there is a time limit between encounters, and a world event were it swims to a spot of the Devil's Roar indicated by a serpent cloud.
I imagine it as an extremely long sea snake/eel covered in a bunch of flamming scales and fins with glowing orange eyes and a bunch of teeth to make it intimidating. It would headbutt, chomp and coil around your boat causing damage and have some fire based attacks like lighting your ship on fire via its breath, and maybe some charged attack too that if you don't shoot a cannon at it in time it incinerates you and deals massive damage to the boat.
It would be a hassle to handle the boat repairs and putting out the fire as well as trying to land shots on it, but once you kill it it would either drop some kind of treasure exclusive to it or just a hefty load of normal treasure to pay off for the hard fight. And if you don't want to fight it you can just sail away from its general spawn area and it will leave you alone, but that's no fun! Just an idea 💡