(Bear in mind this is wishlisting from someone who doesn't own a SoB army yet and barely plays anyway, and really just wants more cool models to build...)
I'd like to see more walkers available for the Sororitas. The Guard have their tanks, Marines have their super-soldiers, I like the idea that the Sisters get to have the cool mecha. It seems right somehow, and also an army of angry power armoured nuns supported by giant freakin' robots is awesome.
As far as specifics for that go...
I kinda wanna see a Sororitas Dreadnought (as in "entombed Sister in a colossal armoured death machine" not "medium sized walker"), mainly because Dreadnoughts are cool. I know there's already the Penitent Engine, but that is quite specialized as a fast melee glass cannon. An actual proper, versatile Dreadnought would be pretty awesome, especially if it has unique options available to it. Mortis-configuration multi-meltas, inferno cannons, heavy bolters and assault cannons perhaps? A flaming censer-fist? Perhaps a "Condemner Flechette Cannon" which fires anti-psyker dart munitions (like a more high-tech version of the old Condemner Boltgun)?
Another walker idea I had was something inbetween the size of a regular Dreadnought and a Leviathan/Redemptor. Not sure if it would be an actual Dreadnought or a manually piloted walker, but it would fulfil the opposite role to the Penitent Engine- as opposed to being a speedy squishy melee unit it would be a ponderous, tough-as-old-boots ranged weapons platform. Big, heavy, slow, but very hard to kill and armed with some big, big weapons. Perhaps give it the "dual kit" treatment and give it a melee configuration intended as a defensive/bodyguarding role rather than a "charge at the enemy" one.
I remember in a mod for DoW someone spliced together the Immolator's turret and the Sentinel's legs and made a Sisters Sentinel with flamers or meltas, and it was awesome. That would be nice.
And finally for silly robot ideas, remember Inquisitor Karamazov? The guy who used to be in the Sisters codex with his big stompy Throne of Judgement? Well why not make Thrones of Judgement as available "mounts" for Canonesses? An Imperium army with a "Monstrous Creature" equivalent HQ choice would be truly unique, and a Canoness riding into battle on a huge robotic throne armed with all kinds big scary weapons would be awesome.
Other non-walker ideas I have include a super-heavy transport equivalent to the Spartan, with an "upper deck" that would allow the occupants to fire without making it open-topped (possibly with a dual-kit option that replaced the transport capacity with a big ol' melta cannon), a Terminator armour equivalent for Celestians and Canonesses (slightly lighter and thus less bulky/ponderous than Termies but a bit less durable) and Repentia with weapon options other than Eviscerators (perhaps big ol' fiery flails?).