I'll agree with the Egghead Light Gloves and Carrot joining the crew.
Carrot was around for so long by the time Wano ended, I just took it for granted that she'd join, like Jinbe. I still miss Carrot, she and Chopper seemed to hit it off.
And the Light Gloves being a Chekov's Gun for later in Egghead just made so much sense that I figured it was a no brainer.
As for some more "unique" theories that have proven to be false:
That Vegapunk would tell us all about Dragons on Egghead during his message. Although, I'm convinced he still knows something he didn't tell us. He was cloning Dragons, after all. This is probably copium, but I still think this theory hasn't fully crashed and burned. One of the engines just blew up, but we're still gliding! I still feel like SOMETHING regarding a Dragon Race is coming down the line.
That the Gorosei were all Joyboy's crew from the Void Century who mutinied against him. Again, this hasn't been fully deconfirmed. But, this one I'm willing to give up hope for. With Saturn's death, it seems pretty unlikely that these guys have been around for the last 800 years. I doubt Imu would get rid of someone who's been with them for the last 800 years so unceremoniously. So, chances are they all did join up at different times, instead of all together like I thought they might. It's possible some version of the original Gorosei were around 800 years ago, I suppose, but that wasn't really the "spirit" of this particular theory.
Here's a weird one, see if you can follow my logic here. I thought that maybe Bonney hadn't actually eaten her Devil Fruit yet. This was before we found out about Saturn's experiments. I thought the unique "time travelling" properties of Bonney's Age Fruit would be used more in her "origin". I thought that, sometime in the future, Bonney would eat her Fruit while she was in her 5 year old body. Therefore, because of the power of the Age Fruit, she would gain the power of her Age Fruit whenever she was in her 5 year old body, anywhere in time. Which means, she got her Fruit in the past because she was just as old as she would be in the future when she eventually ate it. (Could anyone follow that?) I'll admit, this was pretty unlikely just due to how confusing it would be. I just thought it'd make for a fun time travel "joke". Like a parody of how time travel stories work.
I thought that Kuma was visiting Foosha Village, not to check in on Luffy, but because the Ancient Weapon Uranos was actually originally there. I thought that they were going to reveal that Luffy's hometown was going to be where the Ancient Weapon was buried. This was before Imu's weapon was revealed to be Uranos, so I thought the "Mother Flame" was actually going to be revealed to be a new artificially created Ancient Weapon. I thought they were going to make Luffy's hometown special, and maybe make it a target for the Final War.
Personally, I thought they were maybe teasing Sanji turning evil during Wano, and Zoro would have to fight him for real. He seemed to be using his Raid Suit more and more, and it seemed to be corrupting him. His eyebrows were staying reversed longer and longer. But, then that plotline just kind of stopped. I guess it might come back up later, but it doesn't really feel that way right now. So, I don't know if this is still a ongoing plotline or something that Oda just dropped.
I thought Lucci and CP9 would come back as allies for the Strawhats after they left in their cover story. Because it kind of felt like that was where the story was going after that cover story, they left and said they would return. And I guess I just assumed they meant "for revenge". But, either Oda changed his mind, or I was reading some iffy translations?
I honestly didn't think Pedro would stay dead on Whole Cake. If only because there had been SO many fakeout deaths in the manga before then. I kept waiting for him to show back up for a dramatic moment, but it never happened.
For a while I thought Imu was the Mother Ocean/Mother Nature. The entity that originally "cursed" the Devil Fruits so no one who eats them can swim. And this hasn't been fully deconfirmed, but I don't think it's too likely now. I just don't think it makes sense for Imu to live ALL the way up on the Red Line if they secretly have Ocean/Water powers. Why live so far away from the element that you supposedly control? I, personally, just don't think it makes sense anymore.
I thought that maybe Imu was the person who inspired the creation of the Nika Fruit. Basically, I thought "Nika" was Imu when they were younger, and before age and time wore them down and turned them into a different person. A person who had to make SO many compromises and tough choices that it changed who they were. A person who their younger self wouldn't even recognize anymore. This seems pretty unlikely now that we've seen how shaken Imu was just by sensing Joyboy's Haki Knot. Still, I think it would have made a interesting idea. A person having to face off against their younger, more idealistic self.
I thought that Rocks found a way to "edit" or "change" or "create" Devil Fruits on God Valley. Like, maybe there was a crashed spaceship or something on God Valley that had technology that would allow you to change or create any Devil Fruit you wanted. Because, I just found it odd that Rocks's Crew had so many odd and powerful Devil Fruits. I just thought that since Kaido's Fruit was actually a type of Fish Fruit, maybe it got changed artificially into the stronger Dragon Fruit. I know that's based on the legend of a carp turning into a dragon, but I just found it odd from a practical standpoint. So, I thought maybe Rocks would find this Devil Fruit Making Device on God Valley and give his entire crew incredibly powerful broken Fruits. Like Big Mom's Soul Fruit, Shiki's Float Fruit, and Whitebeard's Quake Fruit. I thought maybe THAT'S why the World Government nuked the island. Because this incredibly powerful piece of technology had fallen into the hands of someone who WOULD exploit it to make his entire crew unstoppable.
I thought it was possible that Luffy was NEVER going to find out about eating the Nika Fruit instead of the Gum-Gum Fruit. I thought Oda would just keep the information away from him, somehow until the end of the story. Then Luffy would hear the story about Nika and how he's prophesied to be reborn someday...and then Luffy would go on a NEW adventure to try to find this guy because he'd want him to join his crew, because he sounds like a fun guy. Basically, ending the story with the Strawhats going on another around the world adventure, where they didn't even know that they were already with the guy they were looking for. An endless fun adventure that would never end, because they just didn't have this key piece of information that Luffy IS the reincarnated Nika, already. I just thought it'd be a funny way to end the story. And kind of a fitting way for these characters to sail into the sunset. This seems PRETTY unlikely now that they're on Elbaf, where they worship Nika. Not fully deconfirmed but, Oda would have to work overtime for none of the Strawhats to learn ANYTHING about Nika.
And there are probably a BUNCH of theories of mine that will be deconfirmed pretty soon. But, that's the nature of the theory-crafting game. It's still fun to think about how all these puzzle pieces could fit together.