1. A Kree From wiki: "True Kree resemble humans almost exactly, with the exception of blue skin. Kree have a higher strength level than that of a human, and require more nitrogen to breathe comfortably. Kree bodies are adapted to environmental characteristics on Hala that are un-Earthlike: they have a higher gravity and a higher concentration of nitrogen in the atmosphere. Under Earth's lesser gravity their strength and speed increase. However, they cannot breathe in Earth's atmosphere without using a chemical, "breathing formula", or artificial life-support devices."
Support for theory: The Kree race will appear in the Guardians of the Galaxy movie. The markings on the side of the blue man's chest look
very similar to the insignia on the front of Ronan's armor:
Criticism of theory: In the comics, Kree bodies are not known to produce fluids with healing properties. However, this may be a new addition for the MCU; or perhaps the body has been tampered with in some way.
2. Geheneris Hala´son Mahr Vehl From wiki: "Mahr Vehl is a member of the Kree... He is a "Pluskommander" -- described by him as being equivalent to captain -- in the Kree's interstellar fleet, the Kree Void Navy."
This is an extension of the Kree theory, positing that it is not just any Kree, but the renowned Mahr Vehl...
Support: The letters "G.H." are seen multiple times on the wall of the room where the blue man is contained, and also preface the name of the healing solution which he produces.
Criticism: Would Marvel use a character like Mahr Vehl up in a TV Show? It seems unlikely.
3. Garbha-Hsien From wiki: "Saul (real name Garbha-hsien) is a fictional character in Marvel Comics. His first appearance was in X-Force #10. He is both a mutant and one of the Externals."
Support: The letters "G.H.". Garbha-Hsien is (virtually) immortal in the comics, which might explain how his body is still alive despite enormous damage to the lower half of it.
Criticism: Garbha-Hsein is an External (a subspecies of mutant) in the comics, which is part of the X-Men side of the Marvel universe. Marvel would most likely have to negotiate a deal with Fox to use him.
4. An Inhuman From wiki: "At the beginning of the Kree-Skrull War, millions of years ago in Earth time, the alien Kree established a station on the planet Uranus, a strategic position between the Kree and Skrull empires. Through their work at this station, they discovered that sentient life on nearby Earth had genetic potential invested in it by the alien Celestials. Intrigued, the Kree began to experiment on Earth's then-primitive Homo sapiens to produce the genetically advanced Inhuman race."
Support: We know Marvel wanted to use the Inhumans (there was talk of a film a few years ago). The appearance of Inhumans varies hugely (they can look like anything; any skin color or height, etc - some are certainly blue).
Criticism: Would Marvel introduce an Inhuman before the Skrulls (and Kree-Skrull war)? It seems unlikely, but maybe the backstory could be dealt with further down the line.
5. A Sakaaran From wiki: "A race of aliens from the planet Sakaar."
Support: They are known to exist in the MCU (
toy versions of them have been seen for Guardians of the Galaxy).
Criticism: They have grey skin (although this could be tinted blue by the water).
6. A Centaurian From wiki: "A humanoid race with blue skin... average height of 7 feet (2.1 m). Some demonstrate "psionic" abilities. They are a primitive tribal race, using bows and arrows in hunting. Centaurians are native to Centauri IV, Yondu of the Guardians of the Galaxy is a Centaurian"
Support: They are blue and are established to exist in the MCU (Yondu in Guardians of the Galaxy).
Criticism: As with some of the other alien races listed here, Centaurians are quite an obscure race and (casual) Marvel fans might not recognize the name. Using the Kree would be more logical, as they are popular and have appeared in e.g. Earth's Mightiest Heroes cartoon.
7. A Luphomoid From marvel.com: "When Galactus devoured the planet Luphom, only a small number of of Luphomoids who were off-world invading another planet at the time survive"
Support: Luphomoids are blue and are established to exist in the MCU (Nebula in Guardians of the Galaxy).
Criticism: (see Centaurian criticism)
8. A Sark From wiki: "A humanoid race with light blue skin, standing 6'1" on average. They come from the planet Sarka, third from the sun in the Tilnast star system in the Milky Way galaxy.
Support: Right appearance (skin colour and height).
Criticism: The race is not known to exist in the MCU.
9. A Frost Giant Support: They are blue, and are established to exist in the MCU. It would not be the first creature from Jotunheim to slip through to Earth.
Criticism: The Frost Giants we have seen do not have ears, nor smooth skin. They are typically much taller than the blue man in the tank.
10. An Atlantean From wiki: "Homo mermanus is a fictional race of gilled aquatic humanoids"
Support: They are blue, and can survive in water.
Criticism: The blue man does not appear to have gills. Universal owns the rights to "Namor", and it seems unlikely Marvel would introduce the Atlanteans as a race without him.
---
So, those are all the theories I can think of! If you have any other ideas, post them in the comments section below.