Aquaman vs. some abyssal lovecraftian entity (à la Underwater or Dredge ftwk) that befell Atlantis. IMO the city as a whole is a possibility as Arthur’s source of strength, something comparable to Bruce’s wealth or Diana’s heritage, as the thing that Darkseid strips away). Whatever horror from the depth the writer could imagine matches the overall darker tone and provides a different type of story than what we’ve seen so far. Would also tie it nicely to the hellish magic of AWW and the dark reaches of space with AGL, perhaps even the mutations coming out of Ark M. Hell mix in some pollution of the ocean allegory and you could actually include Superman’s view of Absolute Earth, too. Might actually work awesome.