I remember a few fun ones,
like back in W7, people had Zoro matching up with Spandam, thinking he woud be stronger than what he let on. There were also theories on the toys in dressrosa being their own race, or Sengokus goat being the real boss, pwngoat, but that was mostly a meme lol. I guess the theory of Kaido being a poneglyph who ate "ate" a df isnt unconfirmed yet, but still unlikely, keven more unlikely compared to before. xD
We did a video on the subject btw, if you anyone is interested in a few more theories that never came true, take a look.
5 One Piece Theories That Never Came True