Hmm, "Golden Lion"…
He has some kind of association with the marines and "has been gone for almost 20 years".
Back then, Kuzan, Sakazuki and the unknown named Kizaru were still Vice-Admirals, so they did not have their respective titles, which are:
Blue Pheasant
Red Dog
Yellow Monkey
This could be a clever clue: That guy is a former Admiral.
And considering the fact that even Sengoku is pretty surprised AND he has a wanted poster- Golden Lion did something against the interests of the World Goverment. A perfect enemy for a movie.