User blog comment:Xilinoc/Ch. 621 - THE DARK CURTAIN/@comment-7606564-20150326123608/@comment-5693502-20150326134210

The most likely release we'll get to see is Byakuya and/or Shunsui I think. I feel like Urahara is too much of a scientist to resort to his zanpakuto this late on in the game, the Aizen shikai thing is likely and all the others are B-list and C-list characters so they get no love from Kubo.