A quick explanation for the theory is that there will be 13 Straw Hats in total with each member of the crew will be based off each Zodiac representative. The first big clue to support this theory is when Oda drew the color spread from Chapter 651 as you can see here:
Your initiate thought might be “that’s not the Zodiac except for like 3 of them”, true. That’s what I thought at first too. But if you compare the similarities of the animals in the color spread with the actual Zodiac they actually link up pretty well. We have:
Luffy monkey = monkey
Zoro tiger = tiger
Nami cat = cat (often consider the 13th member of the 12 Zodiac)
Usopp chameleon = snake? (both are the only reptile)
Sanji goat = goat
Chopper red panda = rat? (similar in appearance and being the smallest member)
Robin falcon = rooster (birds)
Franky rhino = bull? (Rhino in Chinese is ” 犀牛” with “牛” literally means bull/cow. Also prior to the color spread Oda had already assigned animals to the Straw Hats in the SBS, with most of them differ from the color spread actually. Franky was assigned “bull” there)
Brook giraffe = horse? (same thing with Franky, he was assigned “horse” in the SBS question)
Now we are only left with rabbit, dragon, dog, and boar.
Jinbe despite being a whale shark will be assigned “boar”. They are similar in appearance from their teeth to the body size.
That’s our 10. So there’s 3 left.
Here’s the prediction for who the 3 new members will be which also “coincidentally” fit well with the leftover Zodiac in my opinion.
First one will be very obvious, Carrot = rabbit. I’m sure the community is well aware she’s one of the more prominent candidate to be a Straw Hat.
The second one will also be one of if not the favorite to be the next new Straw Hat, which is Yamato. Who was revealed not long ago ate the mythical Zoan Dog Dog fruit.
The last one left is the “dragon”.
So who is the first person we thought of when we mention dragon in the One Piece world? Kaido? Monkey D. Dragon? Well there’s this one person who’s also a decent popular candidate among the fanbase to be the next Straw Hat, which is Momonosuke who can become a dragon just like Kaido.
So there we have it! Wait no, I actually don’t think he will join the crew. In my opinion he will most likely stay in Wano establish himself as the next Shogun, the heir to the throne, achieve what he wanted to do.
So who will be representing the Zodiac “dragon” in the Straw Hat? My real answer and guess will be Vivi, the princess who carry the blood of the Celestial Dragon. As from the little we know she’s been in trouble and run as of right now in the story timeline. I believe she will reunite with the Straw Hat, join and be the last member of the crew.
So the Zodiac list would be like this:
Rat = Chopper
Bull = Franky
Tiger = Zoro
Rabbit = Carrot
Dragon = Vivi
Snake = Usopp
Horse = Brook
Goat = Sanji
Monkey = Luffy
Rooster = Robin
Dog = Yamato
Boar = Jinbe
Cat = Nami
There’s also a nice tip bits I just notice in the color spread, Nami is the only one holding onto their counterpart in the picture. Causing distraught to it from what seems to be a race. As from the legend of the Zodiac, the reason cat isnt part of the official 12 Zodiac is because it finished last in the race after being tricked by the rat. The “rat” in the color spread also looks to be the first in race just like the rat in Zodiac did.