Alright, heres what my idea is.
Suboordinates. Basically, I want to suggest that each Leader(Captain, Arrancar #1, Vizard Leader, Quincy Master, Human Aces, Vasto Lorde etc...)
Have the option of having a SINGLE Suboordinate Character. This will be basically an NPC Character that you control. The Suboordinate will have the same race as the Leader, however, their rank will be 3 levels below the User.
Thus,
A Shinigami Captain will be allowed to have a Shinigami(Shikai unlocked) Aide.
A Quincy Master will be allowed a Junior Apprentice.
The Vizard Leader will be allowed a Vizard Suboordinate.
A Human Pro will be allowed a Human Rookie follower.
A Vasto Lorde will have a Hollow Minion.
An Espada will be allowed a Numero. However, this Numero will be a Fraccion, and allowed to have a Reassurreccion.(Much less powerful then Espada's.)
Now, this will not be a seperate character, as much as a supporter. This will allow Leaders to participate in the game more often(Leaders usually hang back and direct) and keep them from being overpowered. The Suboordinate will be unable to rank up(Exception if Vasto Lorde becomes an Espada. In that case, Hollow will become a Numero.)
So? What you think? It makes sense for a leader to have a less aide. This won't be allowing leaders to have hundreds of characters, but allows them to have an actual suboordinate to control. It cannot rank up, and thus, cannot be powerful much, and, because has to be same race, spying is not possible.