I have a multi-picture deal which means I will not only appear in the second part, but they’re planning with me for a longer period of time, but I don’t know details yet, they’re keeping their cards close to their chest – top secret.
Age of Ultron's most recent villain to join the cast, Baron von Strucker played by actor Thomas Kretschmann announced this in an interview. Considering Hydra is supposed to be making a reappearance in Winter Soldier, even if simply in reference to it's leader (see here for details) means that Baron, a villain heavily involved will be returning for future Marvel movies.
His words seem slightly off, and considering his first language is German, was nonetheless extremely informative. Assuming when he says "the second part", he's referring to Age of Ultron (Avengers 2), this could mean Marvel wants to have him play a role in Captain America 3 since he works for Hydra and would be the most reasonable.