Mother of god, they've been teasing GP's death since 2010/2012

Get ur tin foil hats out boyz

Two theories; let's get the easy one out of the way first: Spooky Gangplank was released on 3-16-2010. Most likely a reference to The Flying Dutchman, though the connection to his death is pretty well known at this point, with China, instead of disabling GP, allowed him to be played only with this skin. On to the more interesting theory:

So we have Gangplank, a pirate, who uses a pistol and a sword. He was also, before MF ruined it, the ruler of Bilgewater; some (including him, in his new, new VO) might say he was king. As mentioned before, Gangplank uses a sword, or cutlass, as is more fitting of a pirate, and more accurately, a Bilgewater Cutlass. Conveniently, there is such an item in the game. Bilgewater Cutlass builds into two items: hextech gunblade (not too interesting in this case, though GP does use a gun and a blade seperately), and Blade of the Ruined King.

Blade of the Ruined King was introduced 10-24-2012, along with an updated Twisted Treeline, in the Shadow Isles patch. With the retcon, the Shadow Isles and Bilgewater were placed very close to each other, so much so that last year, GP and MF had to join forces to push back the Black Mist.

The sword itself may not belong to Gangplank; it is mentioned by the altars that the sword belongs to the ruler of the Shadow Isles, not Bilgewater, and Kalista mentions that the 'King' was "a noble king until grief overcame his reason", and 'noble' isn't quite the word I'd use to describe Gangplank. There maybe some untold story where, given the proximity of the two regions, last year's Harrowing wasn't the first time Gangplank ventured to the Shadow Isles, or perhaps (less likely), Gangplank died, his soul went to the Shadow Isles, conquered it, and then came back to life?

Regardless of if the actual sword belongs to Gangplank, the connection of a Bilgewater Cutlass and the Blade of the Ruined King, to the "Ruined King" of "Bilgewater" (Gangplank) is....suspect, to say the least....