I'm astonished Dagoro didn't pick on this, so... I'm calling it: the Yonkou Law is after is Kaidou and the next crew to join Strawhats+Hearts alliance is the Drakes (X-Drake).
Compare the cover page of this chapter:
To the island where X-Drake wanted to challenge Kaidou.
I think this and the chapter talking about a Yonkou is too far of a coincidence to be overlooked. Also, Kaidou is the only Yonkou which overall lackeys haven't been shown except the one shown in the page above.
So the plot will probably go Wano Country-->Doflamingo-->Kaidou.