Admittedly i'm not an expert on captain America or his gallery of enemies however.....
The Red Skull, i'd be more inclined to not go the mask route or the "actual" skull route...
I'd probably go for a the character starting out as a normal man, a soldier picked for the Nazi's own version of the super soldier, however the process goes wrong.
he is indeed blessed with great strength but also his intellect is greatly increased along with his far right views and his desire to serve the reich.
As for his physical appearance i'd have his skin affected by the serum.. It's colour turns a vicious shade of red, the bones in his face become mis-shapen and the skin tightens giving him a skull-like appearance..