The weird rain that Sanji's group encountered a few chapters ago turns out to be seawater spat out by the elephant's trunk - which I think was pretty obvious. I like the detail Oda goes into explaining how the Mink civilization works.
So Jack came to Zou looking for someone - possibly a Mink. And apparently he's dead from the battle we last saw him preparing to enter. But, of course, no body was found, so the laws of shounen dictate that he's still out there somewhere. Still no confirmation that he's with Kaido either, though I still think it's likely.
I wonder if Kaido himself is a mink - it might explain those strange horns of his, and it could possibly relate to the Zou elephant's apparent immortality. Could Jack have been looking for Kaido? There are several horned characters in the story as well - Caeser Clown and the butler Merry from what I recall. Could they possibly have some mink blood?
The relationship of Law's crew to Zou has been explained through Bepo, but there's still some unknown links we don't know of. What was the samurai's goal in traveling to Zou? Why were the people Sanji's group fought back in 795 reportedly looking for samurai? Were they Minks (having been riding the same kind of crocodile as Wanda, some having devil-like horns, and weird animal-like powers) or Kaido/Jack's men? Why did Kin'emon react negatively upon hearing Kaido's name way back before Dressrosa. And why are the samurai being so secretive about this? They said they were going to come clean upon reaching the top of Zou, but being the troll that he is it seems that Oda's going to be making a running gag out of them getting knocked down before they can climb all the way down.
If I had to guess, the Sheepshead person Sanji took out is in fact working for Kaido, horn ability he uses is either the result of a Paramecia horn fruit or an artificial zoan.
After freaking ages, our crew is finally reunited. Almost. Sanji, Brook, Momonosuke, and Caesar (if you can really count the last two) are still nowhere to be seen. And apparently something's up with Sanji, but Oda
Right now I'm thinking that Zou might turn out to be a fairly short arc - right now there just doesn't seem to be any real conflict that's necessarily tied to the island. If Jack doesn't show up out of nowhere, then we might get another 5-20 chapters of exposition/flashbacks before the crew decides to set sail for some reason that will be revealed in that time (right now, looking like it will be due to Sanji). We'll probably get some explanation of the Mink's Electro power too, and maybe there will be some sort of short training/trial of strength arc where the crew is pitted against Zou's warriors (Oda has to be introducing them for some reason). I really hope they're not just being introduced here to be another group in Luffy's army of subordinates.