If Sanji unlocks CoC I think it has to be Wano.
Elbaf is a bit too random. Plus I doubt Sanji gets that much character focus save a matchup with one of the big bads on Elbaf. More likely than not Elbaf will be a Usopp centric arc one in which he might even end up unlocking conquerors Haki but anyways coming back to my initial point, I think if Sanji is going to unlock CoC, it has to be on Wano and one of the main reasons is the potential of Yamato joining the crew. If Yamato does join the SHs Sanji has to have unlocked CoC before then as her existence will undermine Sanji’s status as one of the wings of the PK. Sanji & Zoro are Luffy’s wings bcoz they are his two most exceptional member however Yamato joining brings that into question as not only is she as strong as the two are, she also possess CoC which Sanji doesn’t have. So in the case that Yamato does join, I think it’s fair to question who is more qualified to have that title.
However Oda is clearly setting Sanji & Zoro apart from the rest of the crew so if it’s Oda’s intention to have Yamato join like errone & their grandmum is predicting then Sanji has to unlock CoC.
With regards to how he does it, there are two pathways I can see for him doing it this arc. The first & the most immediate is ‘impending danger’ aimed towards a female character (osome for instance). Next chapter, if Queen decides to dish out his plagues, in a desperate attempt to stop him, Oda could’ve have Sanji unlock it in the same way Luffy, Ace, Doffy & tons of CoC users unlocked it.
The second possibility is Oda having Sanji unlock it in a desperate attempt to overcome his crisis of emotionlessness. For instance if Sanji loses his shit & is about to hurt himself a friend, a conqueror’s burst to dominate that side of him I reckon is feasible.
Will be interesting to see how Oda proceeds with Sanji on this issue ^^