After reading ch. 822 and discussing it with others at the Arlong Park forums, I have a new theory= Usopp will be first SH (beyond Luffy) to meet the WB pirates as he will join Nekomamushi to search for Marco.
As of ch. 822 we know that the SHs will be splitting up. Three will go with Luffy to retrieve Sanji, Zoro to Wano, but what about the others?
Team - mission = known members (possible members)
Luffy's team - to go retrieve Sanji = Luffy, Nami, Chopper & Brook + Pekoms and Pedro (possible Carrot)
Kinemon's team - to go to Wano = Zoro + Law and Kinemon (possible Robin)
Inurashi team - to stay on Zou & protect Momo - ?unknown if any SHs on team + Inu, Momo and Sicilian (possible Franky)
Nekomaumushi's team - to search for Marco - members unknown except for Nekomamushi himself (possible Usopp)
Explanation on possibilities :
Carrot= Everyone is pointing out the obvious here, but to clairify for newcomers. Carrot was the one to say going to Whole Cake Island with Luffy "sounds fun!" and while we see the named minks in ch.822 seeing Luffy & co off, Carrot is glaringly missing. Seems pretty easy bet to say she's snuck away to join the team.
Robin= Thanks to Hudell & uniaka ikuzakas for pointing out that Robin said she wanted to see the ponyglyph on Wano. So she's likely going to join with Zoro going there.
Franky= In ch. 822 he said that he would make some den den mushis. However, he said that to Kinemon's comment of how would the 4 separate teams stay in contact. So he will need to do that before they leave. ITs possible he only makes four before the others set out, then stays behind to make more and possible weapons to use against Kaido.
Usopp = If its correct on Robin & Franky, then this leaves only one SH unaccounted for, Usopp, to join Nekomamushi's team. And I do find it a tad odd that it says Nekomamushi's team, but only show Nekomamsuhi's face. One may be tempted to say he would go alone but the page says 'team' as in more than one.
Beyond the obvious choice of having at least one SH with each team, I have a reason to hope for Usopp to actually join Nekomamushi
WB commander Vista and Yasopp (Usopp's dad) have some sort of grudge with each other. It would amuse me if WB pirates recognize Usopp as the son of Yasopp. This recognition of being the son of a famous pirate is one thing that's been missing for some time. I mean Luffy of all people recognized Usopp as looking similar to Yasopp. So why not others?
Only once was there a enemy/rival of Yasopp to recognize Usopp as son of Yasopp, that was Daddy Masterson. Yes that was anime, but it was originally written for manga but Oda cut it out at last moment to 'make time' so the crew would reach Grand Line by ch 100.