Okay, I'll go first. Picture this guy from "Megamind:"
Now picture a larger and more realistically depicted aquatic life-form, maybe even longer and ending with a tail. The alien is wearing a Starfleet-issue M.A.R.S, a Mobility Accommodating Robotic Suit, which approximates a humanoid, bipedal form clothed in a Starfleet uniform.
His name is Ic'Thus, one of various species from the aquatic world Thalassis. Ic'Thus' species, unlike the Selkies of Pacifica, are are not bipedal and therefore cannot simply wear hydration suits. Ic'Thus is one of a handful of Thalassians to decide to leave their homeworld, and is the first Thalassian in Starfleet.
The Thalassians are a brilliant species, and Ic'Thus' specialty is engineering. He is the ship's chief engineer and enjoys working on the ship's systems more than being with other people, a quality of his species.
I think Ic'Thus could work in any era or iteration of Trek and would offer some new possibilities as one of the alien crew-members through whom humanity is viewed and explored.
Let me know what you think about my character or add your own!