I found a few reasons why the former Spade Pirates may also join the Strawhat Alliance alongside Marco against the Yonkou Kaidou, if you are interisted than read on.
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 Kaidous Commander also called the 3 Calamities are named after card ranks. One of them was allrdy revealed, Jack the drought.
A picture of Odas desk leaked that the other 2 Calamities were called King and Queen.(The note on the top left, the first 4 names are Kaidou, King, Queen, Jack).
Also Kaidous allies seems 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 Kaidou is Doflamingo also known as “Joker”, also the Donquixote Pirates follow the card theme, there are 4 divisions every one of theme 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 Luffys 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 thats simpel, the former Spade Pirates are part of the Whitebeard Pirates and Nekomamushi wants to ask Marco the captain of the Whitebeard Pirates to join the alliance against Kaidou.
So if Aces former crew member take note about it, they will definetly join the alliance to support the little brother of their former and beloved captain Ace, who were 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. (Marco is not Big Mom and I have no doubts that he would let them go)
So the Wano Arc might end up with cool storys about Ace, we also know Ace was in Wano, he learned there how to make Straw Hats like the one he made for Little Oars Junior.
Little Oars Juniors crew might resemble the heart(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 looke like Kaidou is related to the Oars tribe another hint for him to aapear there).
The diamond 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
