You know the hammernado stormcasts have? well, we should have a ****** sharknado.
other than that, maybe some sort of giant tidal wave, that could work for cover and mortal wounding stuff like a better Giant Jaws spell. Maybe a swarm of swordfish that swims around stabbing things like quicksilver swords, or possibly a Kracken spell for area control.
and, as long as we are wish listing... I want Crab knights, a giant skeleton terrain piece of a dead monster, a giant sea dragon monster as a throwback to old elfs, and more giant squids or stuff. Really, there is so many ideas.