Nel or Yoruichi is probably most likely. There might be one or two other possibilities, but they seem a lot less likely since they'd have to find some way to travel there.
Has Yoruichi ever been shown to use a katana? I don't think it's her. It's pretty much guaranteed to be an espada, which means it's either Grimmjow(fan favorite, and editors love to bring back fan favorites) or Nel.
Also more DBZ is always awesome! Can't wait to see what it is.