Since it was pointed out in the chapter 733 discussion thread, the
chapter 693 colour spread does heavily imply that the only unknown character is Kaido - I unfortunately didn't notice that before =/.
To me, that fraction of a glimpse we get of that characters face reminds me of an Elbaf Giant. Putting that image beside Hajrudin's face and Dorry and Brogy's it does appear (to me at least) that the unknown character has a mouth feature similar to the Elbaf Giants.
This would help to explain why Moriah is so obsessed with "Giants" - Oars and Oars Jr. He may feel he needs a Giant to beat a Giant =/.
Law's statement of referring to Kaido as a creature would make sense as Elbaf Giants aren't human. If Kaido as an Elbaf Giant has a [beast-themed] Zoan-type Devil Fruit, I could see where the epithet "Kaido of the Beasts" could come from. It could also come from the fact that he has hundreds of "beast" Zoan-type Devil Fruit users under his command. I suppose an extremely ferocious "beast"/"creature" eating a Mythical Zoan, like a potential Giant Giant Fruit, Model: Titan (or the Typhon model others have theorised about) or a Titan Titan Fruit, Model: Typhon could also make sense. We have Human Human related Fruits, so why not a Giant/Titan related one?
Or just to throw it out there, Kaido might just be one of the experiments Vegapunk and the World Government experimented on to create artificial Giants, especially if Vegapunk were using the Elbaf Giant as a base and experimented with different "beast" parts to strengthen the being and give it raw power equaling that of Giants. If Kaido was a "creation" from an experiment, it wouldn't be too strange for him to rely on experiments again to power his crew - the SMILE.
The only information we have about Elbaf is stories from Elbaf Giants who hadn't been to Elbaf in decades so even they could be unaware of what actually may be happening in Elbaf. It just seem strange that for such a focused on location - revealed early on in One Piece, we hadn't had any information concerning its current affairs at all, this may be intentionally by Oda.
Jaguar D. Saul in
chapter 392 stated that the Giants from Elbaf were "barbaric", probably mainly due to their pride and honor of following through with those "death duels". I suppose not all Elbaf Giants could be like Dorry, Brogy, Omoi and Kashi. With the revelation that there is this "new generation" of Elbaf Giants, it makes you wonder about the "old generation" and what the difference or differences are. Hajrudin did note that he hasn't seen any Elbaf Giant with a Logia Devil Fruit ability, so I suppose until proven wrong, we could take Hajrudin's word for it. Hajrudin also expressed interest in becoming the "King of all Giants", and because of that intention, he competed for the Mera Mera no Mi. If he saw himself needing the Mera Mera no Mi to gain enough power to rise to the top of Giants, then there must be some incredibly powerful Giants out there - Kaido? If we assume Kaido's crew is filled with a substantial amount of Giants, I can see how giving those Giants a Zoan-type Devil Fruit could be concerning and...scary...
If Kaido does have some association with Elbaf, then we may get to see some story of Saul's family, the ones who inherit the "D" get some coverage or at least more information on the "Will of D" revealed. It would seem appropriate to have some information regarding the "Will of D" revealed when a Yonko is in focus - just like how it was with Whitebeard.
It also seem strangely coincidental that when Kaido's names pops up, some development concerning Giants closely follows (Thriller Bark - Oars the 'Continent Puller', Punk Hazard - Giant experiments and the World Governments desire for Giant Soldiers, Dressrosa - Hajrudin and this "new generation" of Elbaf Giants).
Anyway, I'm sure many of us have wondered just how strong a Giant is, making Kaido, one of the Yonko a Giant would be an incredibly interesting development. Just the sheer size of a Giant would induce fear and terror in the common man, combine that with the raw strength Giants wield and the influence and "power" of a Yonko, then you have yourself one fearsome individual. Is this why Doflamingo is so afraid of Kaido? Because Kaido has such a natural strength on top of having enough power and influence to be one of the Yonko =/?
If Elbaf is related to Kaido, it would be one incredibly exciting arc and would potentially have the additional benefit of forcing Usopp into game mode and seeing him grasp a hold of his flickering courage and resolve (alongside his cowardice and fear of course =D). And if Jaguar D. Sual is related in some way, Robin could also have a major part to play (Chopper and Nami had their moment with the Giant kids on Punk Hazard; Franky, Usopp, Sanji and Luffy are having their moment now helping Soldier-san, the Dwarves, Viola, Rebecca, and Law).