Random, but there are some on going themes with one pieces that are mysteries that i'm really interested about.
1. The smiley face jolly roger that many pirates seem to have (possibly influenced from Doflamingo)
2. The whole deck of cards system naming people: Ace, Jack, , Joker
3. Certain characters having what appears to be tusks or horns: Edward Weevil (his mustache), Jack, Kaido
4. Jack and Weevil having the same hairstyle as Lola
1.What do you mean? Are you talking about Law and Doflamingo's JR? My take is that Law, as a tongue in cheek to Doffy, copied his JR while also paying homage to Cora.
2. Something to do with the underworld, or even how Kaido organizes his allies/crew? If it goes by "order" then you have Diez X Drake at 10, then Jack above him, which leaves the Queen, King, and Joker (Doflamingo).
3. The horn thing (real horns or no) is definitely an aspect of Kaido's crew. They all seem to emulate him to some degree as "beasts". As for Weevil, there's a theory going around that Jack is actually his father which is why we haven't seen his face yet (similar moustache maybe?)
4.Yeah this is probably the only reason people are throwing the Jackdaddy theory around. There's also a destruction theme going on between the two. Also, it could explain why Weevil is really fucking strong if he got his strength from his dad.