Please note: these ideas are more or less focused on cool stuff that could be done with SOB, not stuff that a new codex necessarily should have. Be they ideas for wargear, new units that would fit the SOB army, or even just cool special rules that would work with the army, let's give them some love.
Okay, let's get started.
Idea for Acts of Faith: Acts of faith are army wide rules that can be changed at the start of each of the SOB's turns. A SOB army may only have 1 Act Of Faith active at a time.
Shield of faith: All units in the army get a 4++ save.
Undying Resolve: All units with a toughness value get Feel No Pain. All Vehicles gain It Will Not Die.
Wings of the Emperor: All units in the army get Crusader and Fleet. In addition, all units in the army may charge in the same turn as they ran.
Wrath of the Emperor: All units in the army get Furious Charge, Hammer of Wrath, and Hatred.
Suffer not the Witch: All units in the army get Adamantium Will, and may reroll failed Deny The Witch checks.
Armour of Contempt: All units in the army may choose whether they pass or fail any pinning, morale or fear check they are called upon to make. In addition, their leadership is considered to be 10 for any effect that uses that score (EG: Psychic Shriek)
Hammer of Judgement: All units get Preferred Enemy and Rage.
Wargear Ideas: The sisters love fire, so let's give them some.
Combi-Flamers as standard issue weapons to any Sister with a Boltgun.
Purifier Flamers/Heavy Flamers: Special Flamers and Heavy Flamers with the Soul Blaze special rule.
Incendiary Grenades: Assault Grenades with Ignores Cover.
More fun ideas?