I feel like a fun new map Wildcard could throw our way would be one consisting of almost entirely ocean, and I don't mean like The Center, I mean the small handful of islands that would exist would be tiny, like the little bitty sandbar islands between the big islands on Center but no bigger, perhaps with some mangrove swamps in a few places and some ice floes in the north, but very little land. The majority of our time on this map would be spent either in the water or in underwater caves, of which this hypothetical map has an abundance of, in fact the spawn areas would almost all be in these underwater caves. Since scuba gear isn't normally craftable until much later in the game, some new map specific items would have to be included to make going underwater early on more viable, things like a gatherable plant that you could eat to fill/extend your oxygen meter temporarily (like Gilly-Weed from Harry Potter), and some kind of craftable rebreather and some more primitive flippers we could craft from hide and other easy to get materials, also some water-tight structures we could build earlier than Tek-Tier would be a must as well, and make them buildable on platform saddles so we could turn Mosas, Plesios and Tusos into living submarines, and some kind of underwater turret. There would have to be some interesting and varied undersea "biomes" to make this map interesting, things like coral reefs, whale boneyards, and underwater volcanoes spring to mind. A good number of new aquatic creatures would be in order as well, I have some ideas for a few:
Livyatan Melvillei (aka Leviathan Whale): Basically a much toothier version of the modern-day Sperm Whale. In-game it would act much like you'd expect a Sperm Whale to, and ply the deep waters hunting giant squid, or Tusoteuthis in this case. Would be hostile to wild squid and defensive against everything else. When tamed it would have a multi-seater saddle and be a tanky underwater mount that deals bonus damage to squids.
Cryptoclidus (small Plesiosaur): Basically would act like reptilian seals and would be found in shallower water feeding on fish and sunning themselves on the little sandbar islands, or in entrances to the underwater caves of the noob areas. Would be a fairly early tame and I think a neat functionality for it to have would be for it to act like a living refrigerator when saddled, able to store perishable items and keep them at a rate somewhere between a preserving bin and electric refrigerator.
Anomalocaris (cambrian crustacean): 3 words, AQUATIC SHOULDER PET. This guy would act like a sort of living scuba tank for you, passive tame with pearls or fish, gives extended oxygen meter when worn on shoulder.
Helicoprion (whorl-tooth shark): Smaller shark mount, uses those crazy teeth to gather prime meat and/or prime fish meat from corpses.
Archelon (big sea turtle): Immune jellyfish stings and other stun mechanics (except squid grab), tanky, slow base speed but capable of moving much quicker in bursts, gathers increased biotoxin from jellyfish.
Cameroceras (armored squid): A stealth mechanic seems suitable for a relative of modern squids, octopuses and cuttlefish so this dude can camouflage itself and its rider to match the environment for a limited amount of time, scaling with stamina.
Acrophyseter (pods of smaller whales): Another prehistoric Sperm Whale, these guys are a fair bit smaller then Livyatan and found in the shallower waters, can be passive tamed, get a pack bonus to swim speed and damage resistance when you have at least 2, stacking more based on number of whales.
Moeritherium (hippo thing): Found in the shallows mainly by mangroves and hanging out on the sandbars, semi-aquatic animal that gathers underwater plants like the gilly-weed I mentioned at an increased rate (aquatic trike basically).
And of course all of the already existing aquatic and semi-aquatic creatures would be found on this map as well, with penguins on the ice floes, a few Pteranodons, Quetzals, Pelagornis and Tapejara on the sandbars or in the skies, the usual swamp creatures in the mangroves, Carbonemys in the shallows, the occasional Baryonyx on the sandbars, etc.
Anyway I just feel like having more stuff to do in the water would be a fun direction for the game to go if Wildcard ever makes another DLC.