Note: This blog started out as a comment to respond to the excellent User blog:Kaido King of the Beasts/The Nature of the Zou Arc, but it became clear that my "comment" became a beast of its own. You should still check out Kaido's blog cuz it's good!
To me, the answer of where the arc goes is tied to why we're here in the first place.
The Samurai team has a known issue with Kaido, and Zou was known to be their destination. The real question is why Law's crew was sent to Zou: was it just to hide out from the government or something specific?
There's something on Zou that will help the Samurai (and to a greater extension, Wano as a whole), and they seem to have a problem with Kaido. For another reason, Momonosuke needs to be involved in the mission for some reason (why bring a child if the child wasn't needed?), and Kaido wants him captured. This is inferred by Doffy wanting to capture Momo from before Momo ate his DF. Remember that the "3 cards" at play in Dressrosa were 1) Caesar 2) The SMILE factory 3) Momonosuke. Wano has nothing to do with Doffy, but everything to Kaido. If Doffy could recognize both Kine'mon and Momonosuke, and had Kanjuro "captured", and Jack was sent out to deal with Raizo, it stands to reason that Kaido put his best men on this Samurai issue, because they represent a substantial threat to Kaido.
We also need to keep in mind that Law's plan was to trade Caesar for Doffy's Shichibukai position WITHOUT the Strawhats: He had no idea they were coming to Punk Hazard.. Law was preparing for a reality where Doffy was pitted against the Marines or Kaido, and where Kaido was upset with Law as well. His crew arrived at Zou about 2 months ago (17 days ago Jack attacked, The Heart Pirates had been there 1.5 months). We do not know when Law arrived at Punk Hazard, but it's clear that his crew's presence on Zou was part of his plan for the aftermath of the Caesar/Doffy exchange.
Yet if Zou was a direct threat to Kaido, or Kaido knew why people were going to Zou, he would have attacked Zou directly. That did not happen; instead, Jack attacked for Raizo.
You want my theory? I think Zou contains some means to fight Kaido. Whether that's some weapon, the soldiers of the country, or the giant elephant itself, something there can help Wano and/or Law fight Kaido. What ever it is, it's something that Kaido does no expect, or something he cannot have.
You want my totally baseless theory? I think this arc will not end with the Straw Hats sailing away from Zou. I think that Zunisha (the elephant Zou is on) will ultimately walk to more trouble, and the location of the next arc. That might even be the point of going to Zou in the first place.
At the end of the day, the most interesting part of this arc so far is that despite all we've heard and all that we know about the place, we still don't know why we're here in the first place.