i believe robert redford is not playing Alexander Goodwin Pierce and i cant really explain it but i feel in my gut that robert is playing some bigger character
the villains we know in the film are the winter solider, bartoc the leaper, armin zola, and crossbones and all of them are usually sidekicks to the red skull and since hugo weaving said he did not want to return i believe robert redford is playing and older red skull or a new red skull
if he's not playing red skull i believe he's playing baron strucker because baron is a older and
sphociated(sorry for spelling) villain for captain america and redford fits the bill. he can be undercover at shield as pierce to get inside scoop and figure out a way to defeat shield.