It would be nice if someone up-on-high could qualify the "iconic" statement for us a little. If that means "iconic to Star Trek fans", then the villain could be just about anyone that resonates with us strongly enough. Gary Mitchell, Colonel Green, the original Klingon trio, Nomad, drunken shirtless Sulu, whatever.
But since "iconic" in this case almost certainly means "synonymous with Star Trek" to the point where the same non-traditional moviegoers who made Trek XI a blockbuster have to stand some chance of recognizing the name or face... we can probably safely assume it'll be a Klingon, another Romulan, Khan, or an angry whale.