I have to admit, I like Jac's ideas quite a lot here, while also diggin the Pariah storyline from Khaine. Maybe work the two together some how, perhaps instead of falling from grace by allowing a saint to die, have them the objects of scorn and mistrust simply because of their origins, even though it was the Imperium whole made them the way they did, perhaps at the direction of a saint....? Being proud as all SM's are, they eventually grew bitter towards their Imperial masters and rebelled, helping Huron and company during the Badab Uprising and eventually fled to the Maelstrom alongside the marines of Hurons companies. Are also known to make alliances with small warbands from the Cult legions from time to time.
Combat Doctrine: Use surgical strikes by Raptor teams to take out key objectives before dropping a main force to engage the enemy forces and terrorise the population. Use all deamons/cultists shamelessly. Have a particluer hatred (and take delight in killing) for Preists, Inquistitors, Chaplins etc as they see them as ambassadors of the Emporer that never forgave them. They hate Saints even more due to it was the fault of one of them that they fell from favour. They are kinda like a cross of Word Bearers and Night Lords. They favour the use of Close Combat and short range fire fights as it allows them to see the enemy and personally punish them for thier (perceived) betrayal of the Chapter.
I would also suggest that they frequently make pacts with one Runious Power or another prior to entering battle, calling upon the favour of a god in return for the souls they slay. They do not favour one god over the other however, preferring to worship the 4 gods as a single entity.
We should also start planning this legion similar to the rumours of the new codex that will be out in Sept, just to get a jump on it and not have to about face in mid project.
Astral Claws/Red Corsairs and the Dark Talons would be natural allies as well so it wouldn't be amiss to mix squads paintwise perhaps? Thinking too, and I will put this in the painting section of this project, that perhaps any model without a helmet should be painted with eyes that are completely solid colour, dark preferrably without being straight black. Something to show off the geneflaw that would have made them the object of scorn they became.