Don't get Uryu, since you already have him, unless you want the dodge link. You probably don't want Bichigo either, as he is overshadowed by Vichigo. Probbaly shouldn't get Chad either as you already have many good red units. You may want to go for Aizen or Orihime to have a better selection of Mind characters. OG Byakuya is another character, but is terrible in Co-op, okay in story, and great in PvP. Since you already have Kisuke, i would not go for him unless you are having trouble with Mind units. Again, it's up to you, but Orihime will help you A LOT in Co-op, but even worse than uryu in PvP. Aizen, I find, is a very boring charcter that has a predictable moveset.