Here's the thing that one person on these forums best put it a long time ago: GP is essentially a cross between a support and a melee carry. He fits the pirate captain theme to the tee in the sense that he mobilizes his teammates (more specifically his front line) and stands back to let them do the hard work, all the while shooting enemies with his gun and is even rewarded with extra gold for being the one who kills them. Then ofc there's the global ult for global presence and his passive/oranges to deal with any ninja wanna-be that tries to jump him in the back line.
Gangplank's kit has too many iconic abilities that make it difficult to change a vast majority of it. Get rid of the oranges and you get rid of a very unique sustain/escape mechanism (escaping suppress abilities). Get rid of the ult or not make it global and you eliminate one of the last remaining true global ults in the game. Finally, if you get rid of his E, how else will he order around his crew to go and do the hard front line work for him? That leaves him with his Q and passive being the two things that can easily be changed up somehow. How they would go about reworking them would end up heavily dictating whether he ends up being a support/fighter or a semi-melee carry with team buffs.
His E could possibly change up, but ultimately it has to somehow mobilize a crew to retain the theme of a pirate captain that orders his crew around. The ult....well, if that ends up being turned in a regular ult that isn't global, that will for sure piss off a great deal of the general fan base who still desire to see a true global ult remain in this game (Shen is also in the same boat). How Riot addresses the last remaining true global ults will be a sight to see in the future.
Welcome to the forums Roryrai. Feel free to pledge yourself towards the endgoal regarding the destruction of all things casul.