A huge part of One Piece's world-building and ability to maintain rising action is the fact that there are always multiple storylines running parallel to one another. Whether it's the individual arcs for one of its main characters, a massive world event happening in the background, or just a cover story tying up loose ends, One Piece's world has moving parts outside of the immediate story.
However, as impressive as Eiichiro Oda has been in both crafting multiple stories as well as calling back to them whenever they feel important, some storylines do seem to have been forgotten in time. Whether or not Oda is playing the long game or if he's just genuinely lost interest, there are multiple One Piece storylines that the show just dropped.
Gedatsu's Life In The Blue Sea
One of the best parts of actually reading the One Piece manga (as opposed to just being an anime fan) is that the manga's cover pages often expand on the ongoing storylines of a few background characters and past villains.
However, while some of these help build important characters and plots that can't fit in the main story, some cover stories just... exist. One example of one of the most inconsequential and forgettable cover stories goes over one of Enel's Priests, Gedatsu. After Chopper knocks him off of Skypeia, the Ordeal of Swamp lands in the Grand Line and helps an old man start a hot spring resort.
How Law And Bepo Met
It was unsurprising to discover that Trafalgar Law's navigator and kung fu polar bear Bepo wasn't just a random anthropomorphic animal, but a Mink from Zou. While most viewers just took this fact at face value, not many questioned how Law managed to enlist and befriend a Mink before reaching the Grand Line. As revealed in an SBS, Law met Bepo, Shachi, and Penguin not too long after the events of Minion Island.
Bepo himself had actually gone chasing after his brother Zepo when he left to form the Nox Pirates. Bepo would inevitably get lost and joined Law in an effort to one day return to Zou. While it would be nice to see this in story, Oda has stated that he doesn't intend to draw this unlikely friendship.
CP9's Rivalry With Spandam
While it was nice to see Rob Lucci, Kaku, and Spandam finally return to the story, their arrival does leave some questions given their history. In the cover story "CP9's Independent Report," Spandam and the Marines hunt down CP9 after they manage to escape the Buster Call.
When CP9 retreats to their homeland, they're ambushed again by Marines but viciously make quick work of them. This leads Lucci to call Spandam, saying that CP9 will return (likely in revenge). Given how the two parties are now working together as part of a seemingly new group, it's disappointing to see the blood feud and its resolution not carried out on screen, let alone get acknowledged.
Enel's Journey On The Moon
After Luffy defeated Enel at the end of the Skypeia Arc, Enel still manages to carry out his master plan and ride out his golden airship, Ark Maxim, to the Endless Vearth, i.e. the moon. While there, Enel discovers a community of sentient robots who are at war with Space Pirates.
The robots, known as automata, are trying to avenge their creator from the Blue Sea while the Space Pirates are trying to plunder the moon's ancient ruins. Enel manages to overwhelm all of them, scaring away the Space Pirates while earning the admiration of the automata. Enel's galactic conquest, the automata's backstory, the ruins in the moon, and anything relating to alien pirates are never brought up again.
Gin And Don Krieg's Last Journey
Overtaking his captain Don Krieg, it was Gin who had a captivating character arc during the Baratie Arc where his loyalty to his captain was challenged by his gratefulness to Sanji's kindness. In the end, Gin would redeem himself by giving Sanji his gas mask when Don Krieg tried to gas all of the Baratie.
Gin hinted that he was already dying but still intended to continue his journey alongside Don Krieg. After Luffy defeated his captain, Gin would take him away for supposedly one last journey. There's so much wasted potential in not bringing Gin back or, at least, revealing a dying pirate's stoic last journey.
The Straw Hats' Rivalry With The Foxy Pirates
When the Straw Hats first met the Buggy Pirates, it seemed like Buggy and his crew were primed to be the Straw Hats' ongoing rivals. At the very least, they might've taken on a similar role as Team Rocket and become loveable joke villains. However, before Impel Down, the Buggy Pirates chase after the Straw Hats abruptly disappeared from the map.
In their place came "Silver Fox" Foxy and his Foxy Pirates, a group of swindlers who used fixed Davy Back Fights to build their crew. These jokers would go on to challenge the Straw Hats to a few of their games, only to seemingly lose everything. While the anime tried to use them for filler arcs, the Foxy Pirates and their revenge is never acknowledged again in the main story.
Smoker's Chase After Luffy
Smoker used to be infamous among the Marines and the East Blue for never giving up on his prey and always capturing criminals in the end. Like Garp to Roger, it seemed as if Smoker was to be Luffy's Marine archenemy, endlessly chasing him to the ends of the planet. However, after Alabasta, Smoker somehow lost his edge and moniker.
His hunt after Luffy is never seen again; and even after crossing paths with Luffy at Marineford and Punk Hazard, Smoker doesn't follow through after the arcs nor is shown to pursue it again later. He's kind of just another Marine who has a slight grudge against Luffy.
Tashigi And Kuina's Connection
Alongside Smoker, Tashigi's screen time and importance in the overall story has greatly diminished after the Alabasta Arc. This is disappointing given that Oda seemingly teased that she's tied to Zoro's backstory, at least, she looks like she is. During her debut and once again at Punk Hazard, Zoro makes a big deal out of the fact that she suspiciously looks a lot like his dead childhood friend Kuina.
Disappointingly, the story doesn't actually use these opportunities to build a bigger story. Unless Oda always intended for Tashigi's looks to just be a coincidence, there seems to be an entire storyline regarding Kuina's resurrection/twin sister/cloning that disappeared from Oda's interest.
The Poor Village That Tried To Summon Satan
When Bartholomew Kuma rescued the Straw Hats from Marine capture, he sent them all to places that would cultivate their individual skills, with the one exception being Brook. Given that Kuma couldn't possibly know who Brook was, it seemed like he went with aesthetics alone and sent Brook to a creepy community straight out of a Junji Ito story.
The citizens there had apparently been trying to summon Satan in an effort to save their impoverished town; and given how Brook looks, they start praising the Soul King until he gets kidnapped by some thieves from the Longarm Tribe. While it's interesting to see Brook's story progress, he and Oda kind of just left that village to suffer with no obvious resolution.
Sanji's Quest To Find The All Blue
It's not as if Oda or Sanji have forgotten the quest to find the All Blue. It's just that, after the events of Fishman Island and Sanji's multiple attempts to leave the crew for Viola and Charlotte Pudding, the All Blue is no longer a priority for Black Leg.
Its story will likely return at some point; but unlike Zoro's journey to become the World's Greatest Swordsmen or Nico Robin's quest to learn about the Void Century, there is no actual feedback or story of any sort to back it up. Everything else about Sanji's personality (especially the worst parts) have taken over his entire character motivation, leaving the most aspirational part of him to be ignored.