I found a few reasons why the former Spade Pirates may also join the Straw Hat Alliance alongside Marco against the Yonko Kaido.
Beast Pirate and the Card Theme
We already know the Beast Pirates have a card game theme. Ordinary a crew members are named after card games like Sheepshead, Ginrummy and Scotch(Scotch Whist).
And we also know that Kaido’s Commanders, also called the 3 Calamities, are named after card ranks. One of them was already revealed, Jack the Drought.
A picture of Oda’s desk leaked that the other 2 Calamities were called King and Queen (The note on the top left, the first 4 names are Kaido, King, Queen, Jack).
Also Kaido’s allies seem to follow the card theme. We have X Drake who resembles the “10” the highest number in the card game (X is the roman “10” and his father is called Diez Barrels, diez is spanish for “10”)
Another ally of Kaido is Doflamingo also known as “Joker”. Also the Donquixote Pirates follow the card theme, there are 4 divisions, every one of them named after one of the 4 card suits (clubs ♣, diamonds ♦, hearts ♥, spades ♠).
There was another crew which followed the card theme.
The Spade Pirates
The Spade Pirates was the crew of Luffy’s dead brother Ace. So the Spade Pirates are not only named after a card suit, their former captain resembles the last missing high card.
But why should they go to Wano? Well that’s simple, the former Spade Pirates are part of the Whitebeard Pirates and Nekomamushi wants to ask Marco, the captain of the remnant Whitebeard Pirates, to join the alliance against Kaido.
So if Ace’s former crew members take note about it, they will definetly join the alliance to support the little brother of their former and beloved captain Ace who was ready to give his live for them.
It is also possible, if they really appear in Wano, they might ask Marco to leave the Whitebeard Pirates to join the Strawhat Grand Fleet as Spade Pirates with Masked Deuce as Captain.
So the Wano Arc might end up with cool story about Ace since we also know Ace was in Wano, he learned there how to make Straw Hats like the one he made for Little Oars Junior.
Could it be that the Whitebeard Pirates will resemble the card suits, based on their Jolly Rogers? The Spade Pirates will resemble the Spades.
Little Oars Juniors crew might resemble the Hearts (People may ask what about the Heart Pirates? The answer is there is no heart on their Jolly Roger).
Why would they go go there? The answer is simple. Little Oars was a friend of Ace (It also looks like Kaido is related to the Oars tribe, another hint for him to appear there).
The Diamonds might be resembled by the Gamba Pirates.
Why should Gamba go there? Well, he seems to be a strong swordsman, so why not?
And the club might be resembled by the Whitebeard Pirates themselve or another crew with an unknown Jolly Roger.
*Theory by Nessos