Byakuya vs. Kenpachi Zaraki This is the one thing I really want to see because:  
1. Byakuya was designed to be as invulnerable as possible seeing as he was the main antagonist of the first arc.  
2. Zaraki was designed to be so strong he didn't need shikai or kendo or the full extent of his skills. And now he has shikai and the full extent of his skills  

Even in the first arc, there's a scene where Zaraki challenges Byakuya, only to be taken away by Gin. Later, Kubo traps them in Huenco Muendo because he cannot imagine them having a hard time with the guys in Fake Karrakura town. Or he didn't want to impede on their OPness.

I don't know how Kubo would run something like this, though.