It certainly isn't a conventional choice, though MODOK is an original villain of Steve Rogers from the 60s era comic books. This is what Christopher Markus had to say about the character and why he wants MODOK in the movie.
"I love MODOK and I think you could make a terrifying movie with MODOK but nobody seems to be on my side at the moment. I will win you over to Peter Dinklage as MODOK. If he came around the corner and you saw him floating there you would be terrified. It would be amazing. (They) really pulled off the Red Skull. It doesn't look like a mask. It looks like it's fully integrated into his flesh. And I want to see what they can do with MODOK."
In other Steve Rogers related new, The Avengers director Joss Whedon has revealed that he will open that ensemble superhero film with an old newsreel from the 40s featuring actress Joan Leslie as Steve Roger's girlfriend of the time. The director will digitally recreate the departed actress using footage from the film This is the Army.
Captain America: The Winter Soldier was released April 4th, 2014 and stars Chris Evans, Samuel L. Jackson, Scarlett Johansson, Robert Redford, Sebastian Stan, Anthony Mackie, Cobie Smulders, Frank Grillo. The film is directed by Anthony Russo, Joe Russo, Joss Whedon.
Marvel's The Avengers was released May 4th, 2012 and stars Robert Downey Jr., Chris Evans, Mark Ruffalo, Chris Hemsworth, Scarlett Johansson, Jeremy Renner, Tom Hiddleston, Clark Gregg. The film is directed by Joss Whedon.