This is something that has been going on in my mind for so long. First, all of the Yonkos, except Big Mom, have a connection to Kozuki Oden.
Whitebeard saw Oden as a younger brother.
Shanks admired him
Blackbeard: Oden was the one who recruited him into the second division.
Kaido fought Oden or even killed him.
And all big names like Rayleigh, Crocus, Buggy, Marco and many others know him …but why… why hasn’t anyone come to avenge him? We haven’t seen anyone of the big names except Sengoku mentioning him in the present time.
It’s like they don’t even remember Oden. For instance when Robin asked Rayleigh about the words engraved in the Poneglyph on Sky Island, Rayleigh mentioned only about the Voice of All Things. We first thought that the “Voice of All Things” is an ability by which Roger understood the Poneglyphs but no, it’s just a voice that it emits and Roger could only find the Poneglyph’s location with that. Oden was the one who deciphered all the Poneglyphs for Roger… but Rayleigh didn’t mention anything about Oden when Robin asked him…why?
Can it be the work of the memory fruit which is currently used by Pudding? Could the previous user have wiped the memories of Oden from the heads of the big names? Could this be the debt that Kaido owes to Big Mom?
*Theory by MonkeyDEzio