We are all aware of Cross Guild already joining three warlords: Buggy, Crocodile and Mihawk. In all seriousness, Crocodile and Mihawk joining is incredible per se, but I believe there may be a build up towards a group of 7 Shichibukai joining:

Moria seems to be still alive and probably is going to be freed up. Perona is a good link with Mihawk.

Boa has to make a move to protect Kuja, there are many possibilities, but I don’t see it too far-fetched to have her join with Buggy (who helped Luffy), Crocodile (who was Luffy’s enemy, but who also helped him afterwards) and Mihawk who now has no interest in directly being against Luffy.

I agree that, given the possibility to choose Boa would just side with Luffy, but I think that given the circumstances it isn’t so easy for her to come in contact with Luffy right now, while I suspect Cross Guild is actively going around and meeting with powerful people.

Weevil can be a wildcard, but if he has to rejoin the story in some way I believe it can be within the ranks of Cross Guild. I would find it extremely funny. Now we are at 6, who would be the seventh?

I doubt it’s going to be Blackbeard, for obvious reasons, and Jimbe can’t be either. So we are left with Kuma, Doflamingo and Law. I believe Kuma to be unlikely. He will either die or be properly set free and autonomous and in that case I’d say he will remain with Revolutinaries.

Doflamingo is (within this theory I mean) likely to be a candidate, but I also see him one of the worst people and I can’t really see him teaming up with Buggy, or Boa. I’d admit his return and bickering with the likes of Crocodile and Mihawk would be endearing. Also, we know he knows something, or perhaps even more. If he is to return in some fashion and he isn’t dead this may be it.

Law is going to get out of the race for the ONE PIECE, BUT at the same time I am unsure of his demise and death. If there is a way to keep him both alive and part of the story, that may be with him joining Cross Guild. If anything else they may have saved him in the nick of time from an exhausted Blackbeard who managed to get his hands on Poneglyph copies, but didn’t manage to secure the kill and steal the fruit. Of course Law in means Mingo out.

And, well, this! I’d find it really cool if at least 7 of the Shichibukai were to oin together and actually be part of the same crew. In a sense such crew would mimick the Rocks one and in any case I would like it to happen!

So it’s both wishful thinking and a bit of possibilities thrown in the mix.

Theory by AkagamiBarto (https://www.reddit.com/user/AkagamiBarto)