Now this will be a short editorial, and one of my first so go easy on me.
I know not nearly half as much as most the users on this site will know about the Marvel universe, but I think all the evidence points to the G.H "alien" at the end of the last episode of Agents of Shield being an Atlantean.
Now, if this has already been theorized then let me know in the comments and I'll remove the article, just I couldn't find on the site regarding it. So here goes.
1. The being is submerged in water (figures!)
2. Check out those white markings on the body. At first I thought it may be on the glass surrounding the Atlantean but as you can see from the first image it's clearly on it's chest.
3. The long fingers / Nails. As you can see from the images below the Atlanteans have long-ish nails in the comics, also something to note one notable Atlantean as shown in the image below is called "Govan". Could this be the G in G.H? If so what could the H stand for?
Lastly a small point to make, the Atlanteans are a genetic offshoot of Humans, and as you can see this being has the same anatomy, and by the looks of it internal organs as a human, perhaps why they specifically showed half the body including organs?
Anyway that's my theory, and I'm sticking by it!!
Let me know what you think/correct me in the comments!