What they need and what I want... hmmm..
A lot of anti-psyker rules and equipment.
Like a relic that any psyker in range that rolls doubles when manifesting a power suffers perils of the warp.
Anti-psyker grenades
New elite unit, Witch Hunter Squad. Unlimited Condemer boltgun shots with the sniper rule! Infiltrate and stealth with a 4+ save.
Sister scouts would be another troops option. 4+ save, infiltrate, and still take 2 weapons, etc.
I also like the "cultists" idea. 4-6 points a piece, lasguns, las pistols/CCW, options for flamers/storm bolters.
I think Acts of faith should be universal and able to fire off every turn. Or have some that are universal and others have specific.
Retributors should have an act of faith that allows them to have skyfire that turn. BS 4, skyfire multimeltas are scary!
Cannoness could have a bunch of acts of faith that super boost the squad that she's with. And also huge bonuses to fighting psykers in challenges. Like gains perfered enemy, armourbane, and flesh bane against psyker characters.
Shield of faith could also give them a 4++ save against wounds and glancing/penetrating hits caused by psychic powers. Including attacks from force weapons.
Someone mentioned Repentia as troops with an HQ choice selected. Yes please!
Repentia with more weapon options would be good too. A mix of chain weapons and I5 would make it really neat to play. Allow them to take 2 chainswords or a heavy chainsword or an eviserator for different point costs.
Allow sisters to take a land Raider Redeemer.
Make the Immolator a fast vehicle.
Celestians just need a 2+ save; they all have artificer armour. Give them bolter, bolt pistol, and CCW as base equipment. Allow 4 models to take weapons like power weapon/fist, storm shield, lightning claws, special weapons, heavy weapons, etc. Make them tough and versatile. Now that's an elite choice!
Have a Psykerhammer, like a daemon hammer, but works against psykers!
I like the 0-5 Hospitallers in the army. Don't make them characters but allow them to give the unit feel no pain.
Have Ordo Hereticus Inquisitors as an HQ choice. Maybe can't count as warlord?
Or maybe have some other the other Ordos availible. Like Order Sicarius (that could enable 0-1 assassins), Ordo Scriptorum that gives mission bonuses.
That's all I can think of off the top of my head.