Created by Stan Lee and Jack Kirby (as if you couldn't tell), MODOK was originally George Tarleton, a tech for the terrorist organization Advanced Idea Mechanics. He was altered to become MODOK and he eventually becomes the leader of the organization and a reoccurring villian for Captain America.
Interesting. I never thought about using MODOK. I've thought of Crossbones, Madame Hydra/Viper, Taskmaster (which would be cool), Baron Strucker, Baron Zemo, but the obvious choice is Arnim Zola since he was in the first movie.
So what are your thoughts? Who do you wish to see Cap go up against in the sequel?