In a more discussion note, it's interesting that last few arcs had Luffy + Shichibukai and this is Luffy + Shichibukai. I think they are figuring at this point Luffy's power level is so high that the only allies he can team up with (like Vivi) are Shichibukai tier or higher. I see a team up arc with an Emperor at some point in the future.
I'm guessing they're going after Big Mam? If it was Blackbeard Luffy would have gone berserk, and Shanks he wouldn't have agreed to. So it's either Big Mam or the other guy that I'm forgetting. It also makes sense from a writing perspective because on their own, the Strawhats might not be strong enough yet to take on an Emperor. But with Law strong abilities and smart planning, they should have a chance.