Beware of unmarked spoilers.
The Identity of the Bigger BadSomeone from the Bleach world is manipulating things from behind the scenes and is sending Arrancar and Espada to the Hero's world. They are also the one behind the alliance between the Arrancar and the League of Villains.
- Aizen. All-for-One's last words of "He was right about you, Hitsugaya-taicho." imply his "friend" is an enemy who knows Toushiro more than just in passing. Aizen is the obvious candidate. Soul experimentation is right up his alley and since a chunk of Toushiro's PTSD is due to him, facing him will be troublesome (and maybe flashback-inducing) for Toushiro.
- Yhwach. He could have survived the war.
- Mayuri. Immorally finding out more about Quirks by performing experiments on people who have them is something he would do. He (like Aizen) is also one of the people responsible for Toushiro's PTSD so why not make him an antagonist Toushiro has to confront?
Strong reaction to brainwashingToushiro will meet Shinsou and find out about his Quirk (or Shinsou will use his Quirk on Toushiro or one of his friends). Cue freakout.