Gangplank is a Champion that has been in the League of Legends since the very beginning, and yet he STILL doesn't have a true Legendary Skin. Special Forces Gangplank came very close to this mark, but not quite.
I believe that the popular demand for Legendary Gangplank Skin would be one that is science fiction/space themed. The rational for this reasoning is rather obvious:
Gangplanks first ability has him fire a gun. Laser gun and laser animation replace regular gun and bullet animation respectively.
Gangplanks second ability has him eat oranges. Possibly no need to change this, however perhaps instead it can be replaced by an Orange flavoured beverage.
Gangplanks third ability has him raise the morale of his teamates, increasing their movement speed and attack damage. In this skin Gangplank releases a visible Morale Droid that remains for the duration of the effect.
Gangplanks fourth and ultimate ability has him signal his ship to fire on any point in the map. In this skin, Gangplanks signals his ship in space to begin orbital bombardment on any point in the map. Instead of cannon balls, energy charged bolts or MAC rounds hit the area and leave behind glowing holes.
Gangplanks overall apperance would look futuristically rugged. His sword would be replaced by an energy blade (Which I would love to see an option for changing colour based on the users preference, but I know this is not possible); and as with all other Legendary Skins, this Space Pirate Gangplank would have new animations, emotes, and voice acting.
Thanks for reading! There are probaly a hundred more of these threads, however this is not an idea that should be left gathering dust. Riot please!