Well seeing as his shell (skin) was so hard, so resistant, so durable that he had survived Kirinji's hot spring the longest of all the ones that used it, the turtle sounds pretty accurate. Of course if Kubo will go the same route he went in SS arc, then Byakuya would have been the Dragon, and Renji the Tiger, because Tiger is the only being that can stand equal to Dragon, at least there is a saying like that (which was initially Renji's aim and at SS arc Byakuya used a metaphor that alluded to Renji only reaching the reflection of the moon and not the moon, of course then he managed to touch him with his sword :P).
Originally Posted by Anticrobotic
Anyway, these gods were (Chinese, then Japanese names):
Qīnglóng — Seiryū (The Azure Dragon, representing Spring and Wood) to the East;
Zhūquè — Suzaku (The Vermillion Bird, representing Summer and Fire) to the South;
Báihǔ — Byakko (The White Tiger, representing Autumn and Metal) to the West;
Xuánwǔ — Genbu (The Black Turtle, representing Winter and Water) to the North;
Huánglóng — Kōryū or Ōryū (The Yellow Dragon, representing the Changing Seasons and Earth) to the Center;
the 5th one rarely appears :P.
So as you can see:
The Azure Dragon - would have been Byakuya, since wood can be associate with trees and trees with sakura petals;
The Vermilion Bird - would have been Ichigo, since it is suggested that he might have a flame element, beside he already once stopped the phoenix spear;
The White Tiger - would have been Renji (his new clothes even have tiger straps), since his shikai is Zabimaru - Steel Snake, thus associated with metal;
The Black Turtle - would have been Rukia, since her element is ice and the turtle is associated with winter and water;
The Yellow Dragon - if it would have appeared - would probably be Ganju, since he was the only one to showcase a great potential and capabilities in wielding the earth element.
Also, sometimes they are also associated with certain traits and colors, like:
Dragon - Blue - Bravery (symbolizing Byakuya eventually overcoming The Fear Quincy ?),
Phoenix - Red - Kindness (Ichigo showed it multiple times),
Tiger - Green - Strength (yep pretty much Renji, especially the strength of his resolve and will),
Turtle - Violet - Versatility (I guess she can use her Zan, Attack Magic, Defense Magic, Trap Magic and Healing Magic and the like ?).
Speaking of the phoenix spear, I wonder if we will see it later on? Maybe they unseal it, give it to Ichigo, and finally Kill Bach and take his Soul and anything Quincy with him? Zero that makes sense, but I'm kinda sad, I always like the idea of Renji surpassing Byakuya. I could see Kubo using a Chinese Mythology route for towards the end, it seems like he's reflecting off of Buddhism also.