I propose that the ‘Lurking Legend’ that Oda was teasing us about could be, Shiki the Lion.
Now, Shiki the Lion is a canon character considering that he was mentioned in the actual story itself as he was the first to escape Impel Down.
Oda has said that he will introduce this ‘Lurking Legend’ and that he might be related to ‘Whitebeard’.
Some say because of the relationship to Whitebeard that it has to be Weevil, but since Oda says “introduce” I’m going to assume that it was someone who didn’t have an introduction yet.
Shiki the Lion was only mentioned, he has yet to get a proper introduction into the story itself. On top of this, when Shiki escaped he went into hiding, meaning he fits the whole ‘Lurking’ role completely.
Now, what would be his relationship with Whitebeard? Well, Shiki the Lion was one of the top 3 Pirates of his generation, just under Gol D. Roger and Whitebeard. He’s a sly and devious guy trying to make allies with whoever he can and crushing rivals with overwhelming power (As seen in the Edd War).
It makes sense for him to have tried to ally with Whitebeard, and when that failed he would view Whitebeard as a rival that he needs to get rid of.
The relationship with Whitebeard doesn’t have to be Familial or Romantic, in this context it still works.
So, in short. Shiki the Lion is a Lurking Legend with ties to Whitebeard and Gol D. Roger that has not been properly introduced in the actual story yet, so he fits the extremely vague criteria that Oda has given us.
*Theory by Numbuh24insane