The reason Kaido is in Wano is because...
...perhaps, the reason why Kaido is there because it's Wano is because Wano itself is the DF that Gorosei speak about?
The missing 4th animal, the giant turtle that has Wanokuni on its back? Only such thing could be a legend for Gorosei.
Something that hasnt awakened for centuries. However, nothing has awakened now but Momonosuke might awaken it.
Or the reason why it has awakened precisely now is because Zoro just awakened his CoC?!
I believe it's a Bird (The one on Kozuki Crest)
It shows The Bird & inside is Big Circle surrounded by 8 Smaller Circles
Well.....
If you really keep looking at Flower Capital, you will notice that it actually looks like a Bird Nest
And that Tree in the middle is where the Bird stands (That's why Oda made it stretch above the Castle)
So if Flower Capital is that Big Circle with an Ancient Bird ruling over it
Then Eight Small Circles = 8 Regions (Which are the Lands that got connected to Flower Capital)
However there are only Five & that's because Wano got Closed (So the Project wasn't Finished)
Which is what "Open Wano Borders" means, it refers to "Expand Wano Borders or Finish The Project"
Hence why it's called "Country of Harmony", cuz it's created to be Home of Multiple Groups (Each accompanied by a Titan Animal)
Which is also why Ports of Wano are named after "Animals"
So the reason Zunisha is coming to Wano is because That Bird has Awakened (Ruler Bird), who can call other Titans
This is why Gorosei started saying that it's impossible "It Awakened", cuz Zunisha would only come to Land if it was Called !!
(All of this is inspired from Mothra & other Kaiju who in Lore represents Protector of Japan & Earth)
Perhaps the Crime that Zunisha did is causing this Bird to fall asleep or die or disappear in the past which caused this Project of Wano to Stop, and this is why it was ordered to walk Eternally until The Bird awakens again & calls for Zunisha to come back (Which is the Event that Toki was searching for to witness at first).
As for the other Titans, either they are also in slumber or they disappeared too & only this Ancient Bird can bring them back (Which explains why in the Past there were Eight Moons, but now there is only One !!! This Bird represents Earth, so those other Titans represent The Moons, so since only Zunisha is Left, that's why there is Only One Moon Left). And this also explains why every Royal Family from each Region is named after a Moon.
But what caused this Project to Fail? Obviously it was the Kurozumi Region, because while all Wano Royal Families mean "Moon", only Kurozumi mean (Black Coal), so apparently when that Part of Wano was connected, it actually carried something or someone who didn't belong with Wano & led to Disastrous Events, or should i say, the Titan they brought was a Fake !!
If we check the Port in Kurozumi Region, it's actually "Itachi" which means "Weasel"
Conveniently Weasels are known for being Opportunists & also "Take more Food that they can eat" !!
Which is how Yasuie described Orochi for example & also it shows how Kurozumi Region ruined everything & made people Hungry & Poor.
Google Weasel's Bad Reputation to see more about this
I don't think this means that the Titan who ruined Wano Project is a Weasel, but it simply refers to their Behaviors.
Anyway, all of this leaves us with just one question, who awakened this Bird??
I believe answer is very simple, Onigashima is approaching Flower Capital, so as Protector, it's time for this Bird to Awaken & stop this Catastrophe
But where does this Bird come from?
Well, it's most probably The Egg that was on Roger's Ship, which i believe Roger found on Lodestar Island
And that is where he learned about some Important History & why he was interested in meeting Oden & ask him to join him, even though he never went to Wano at the time & shouldn't have known that Kozuki Family can help him with Poneglyphs