I agree with the Oni DF theory too. With Oda's tendency to link his tributes to folktales and mythology, with Momonosuke being a reference to Momotaro, it seems very likely for Kaidou to play the role of the Oni King. With his size and horns, he could be from the same giant race as the Oars family too.
The Momotaro reference isn't completely likely though, since Sentoumaru shares inspiration from Momotaro, along with Aokiji, Kizaru and Akainu all being linked to the tale, yet I find it doubtful they'd play a major part in the Zou or Wano Arcs.