If Gangplank were to receive a Legendary/Ultimate Skin, it should be Space Pirate-themed!
Think about it guys. There is no way he could expressed in any other possible way... Kind of.. But that's not the main point! The level of badassery that could be shown through this Space Pirate-themed skin is unbelievable. Here me out guys...
Envision this. His pistol.. gun... or whatever it is... Ditch it. Think better. Think cooler. Think space. Now you see Outlaw Star. Now you see the Caster Gun. Now you see Gangplank wielding a variant of a Hellfire handgun because Pulsefire is out of style. Now, you're thinking to yourself, how could it be complemented? Well, imagine you playing No More Heroes. Next playing Megaman Zero. Then playing Xenoblade Chronicles. And then finally playing Halo. You say to yourself, a plasma blade which could replace that measly old cutlass. Good riddance!
What's next? You're now Gangplank destroying Top lane because you don't poison people anymore... YOU BURN THEM AKA PLASMA/ENERGY BURN THEM!!! You're enjoying the screams of the enemy team's Top laner and their Jungler... until the Jungler flashes away... How shameful. You have Gangplank raise that Hellfire gun in the air as it transforms into a propulsion device that sends a shockwave, enhancing your weapons and speed to execute the landlubber. Now things are fine. That is until late-game comes and you see your teammates getting beat up by the enemy team because they are Nooblords. But you're courteous. You press that R button with a precise motion and you see Gangplank raise his Hellfire handgun yet again into air and see that beauty transform into a cool looking antenna that relays a signal to his ship. You turn your view towards the enemy team as they perish under the Dead Pool's devastating light. You aced the team. You destroy the nexus. And you then eat a Citrus and everything is K.
So that is why if Gangplank were to receive a Legendary/Ultimate Skin, it should be Space Pirate-themed!