From what i understand zaraki has taken the sword from Unohana who's bankai had the effect of everything melting down and becoming liquid. Now my theory behind her bankai is that they don't necessarily melt but actually become in a state of complete relaxation where their entire body is loose. It is said that complete relaxation of every part of the human body can increase physical attributes to their maximum potential.
No, it doesn't seem like he took a sword from Unohana.
He seems to awaken his own Zanpaktou that started talking with him or rather it's better to say that Zaraki finally managed to hear its voice. So we are obviously bound to see his Shikai at least. The thing is what is his power. It would be funny if it was Kido-based since starting from Zaraki those with Kido-based Zans weren't looked good in 11th Division.
Ohh it would be awesome if it is Kido-based xD, and maybe using Urahara's device he may learn bankai too?... Yoruichi is missing from the battle after all, she might be on it
Well, if Zaraki gets Bankai, then he might kill half of Sternenritter by himself, so I don't really know if Kubo will go this way. He is already damn strong. So giving him Bankai would be a strech. Shinigami need to have real problems during this War. And Ichigo should also take part in action.
Shikai = yes, Bankai = 50/50. But given that it is Zaraki, he's probably have learned his Bankai with his own "training".
Hm... If his shikai ability would be like... the bloodier the sword gets, the stronger he becomes... that would be cool. Would suit him much.
Bankai? Release all the stored (blood-)enegry in 1 huge slash/impact/cero-like something. This way his power would be fairly awesome in shikai and his bankai would not become instant overpower to everyone. Or at least he would need to fight before going bankai and pwn anybody. This way it would still be beleiveable that he is not God.
Here's an idea!!!...
So Zaraki's Zanpakutou has the ability to amplify his swing to the required level to cut through an object. When Zaraki's blade initially makes contact with the object, the attack will have the same power as he put into it. However, as long as he keeps the blade in contact with the object, the attack's power will increase exponentially until he slices through the object. As soon as his blade loses contact with the object he's cutting, his attack power returns to normal. This ability applies to both his Shikai and Bankai; the only difference is the rate at which the attack's power increases...
So let's put a specific number on his Shikai; we'll say that his attack power will double for every second that his blade is in contact with an object. This would mean that if Zaraki swung his sword at an enemy and kept his sword in contact with them for 2 seconds, his attack will have amplified to 16x its original power!!! There are a few disadvantages to using his Shikai, however. Firstly, he must keep his blade in contact with the enemy in order for his attack to be amplified; if the enemy were to break contact for even a split second, the attack would be 'reset'. Secondly, the amplified attack would use a corresponding amount of Spirit Energy, so Zaraki would end up wasting a lot of it if the enemy kept breaking contact with his attacks.
So now lets look at his Bankai; well, this would be very similar to his Shikai, apart from his attacks would be amplified instantly!!! In other words, his attacks would contain the exact amount of force required to cut through whatever he swings his sword at. Unfortunately, this would have a deadly side effect; if Zaraki were to attack an insanely tough enemy using Bankai, he would instantly be drained of a huge amount of Spirit Energy. If he tried to attack an enemy that's GODLY tough using his Bankai...well...Zaraki would be the one who'd die from the attack when all of his Spirit Energy is sucked out of his body!!!!!
If zaraki does have an ability it has to be something which can somehow be connected to his previous fights. I don't think kubo would waster a chance to connect all his fights to a potential new ability. his ability has to be one out of two things IMO and it necessarily has to be something relating to how he regularly fights. The first thing that comes to my mind would be zaraki's unnatural resistance to injury. Basically his zampakuto could be something that heals him and maintains him in top form. It would explain his survival of impossible wounds even by shinigami standards. And it would kinda suit the chapters in which he fought which were titled the undead. The alternative would be what others have suggested, that he gets stronger as he accumulate wounds and damage. Basically every wound he takes would contribute to his sword releasing greater reiatsu. Perhaps it would even account a bit for how his fight with unohana made his stronger, as he died and got healed he would have had his sword forcibly draw out more of his latent power. I lean to the injury healing ability though, it explains better earlier fights.
I would find Zaraki showing a bankai during the upcoming battle to be a bit ridiculous. Besides the obvious difficulty in obtaining a bankai, even with Urahara's method, there is the fact that he has yet to even master a shikai. This progression would be a bit too rapid within the available timeframe. The other objection would be that there wouldn't be any justification for Zaraki to attempt to learn bankai in the time available; the Gotei 13 are facing opponents that are capable of stealing bankais, it would be seemingly counterproductive for Zaraki to use the little time available to improve himself on an ability that would likely be stolen and then utilised against him.