Agents of SHIELD has been airing since fall 2013, and has shown viewers several marvel super villains in that time, some more recognizable than others. Some of those super villains include Deathok, Absorbing Man, and Mr. Hyde. These are all pretty cool B, C, and D- List Marvel Villains. I think that the addition of a more widely known bad guy that poses a genuine threat to the world would be great for the show. Also, the villain I have in mind has already been introduced in the Marvel Universe, and kind of left hanging. [Agents of S.H.I.E.L.D.](tag:722469) could bring that villain back like so many people would like.
At the end of the Incredible Hulk, after Dr. Samuel Sterns helps Emil Blonsky become the Abomination, Blonsky throws Dr. Sterns, and some of the Hulk's blood drips onto a cut in Dr. Stern's forehead. His head then begins to sort of bubble, leading to give a creepy grin. No more is seen of Dr. Sterns, much to my disliking. As most comic lovers know, Dr. Sterns goes on to become the super villain known as the Leader, one of Hulk's most famous foes, and that is who I think should be on Agents of SHIELD Season 3.
The Leader is another gamma powered villain. However, his power is in his mind, not his body. He is given the power of super intelligence that enables him "to think on a level that normal humans can't even begin to comprehend." Also, as you can see in the image above me, the Leader has green skin, and a ginormous head. Now, for why the Leader should show up on Agents of SHIELD.
Viewers were left hanging at the end of the Incredible Hulk, and many people were expecting that a second Hulk movie would be released with the leader as its main villain. However, that second movie never happened, so i am left to think...where is the Leader now? My theory is that the Leader has been imprisoned by SHILED all this time. He has been sitting in his cell for 7 years, waiting for his opportunity to escape, calculating every possibility. Somehow, he escapes, and then Coulson, and SHIELD, go after him. Maybe the Leader goes after other super powered beings, like he so often does in the comics. That could lead to Carl Creel, the Absorbing Man, showing back up and causing some more crap for SHIELD.
The Leader could also make those rumors that an Avenger will join the Agents of SHIELD for at least an episode or two a reality. Specifically Bruce banner the Hulk. Now, I think that they could leave the Hulk out of it, but Dr. Banner could be used to track the Leader, much like he did with the tesseract in the Avengers. He could also be charged with the idea of finding a way to stop the Leader from whatever it is his goal might be. And if a little Hulk is required...so be it.