I have a lot of ideas for the story/stories behind my army/chapter. At this point every time I read a new codex I come up with a little more. The Chorus are the purists from Hell - they don't trust their Techmarines because they run too close to being heretical with their obsession with the Omnisiah, they feel the Grey Knights are corrupt in their use of daemons and hypocritical in their use of xenos technology. They feel that the Imperial Guard has some hope, but pity their weakness and their frail bodies... but that most other Space Marines, even the Dark Angels have fallen to some form of corruption or another.
Story ideas though... the main one being their formation - a strike force of the 8th Company of the Guardians of the Covenant came upon a joint IG/Inquisition training group who'd been fighting a guerrilla war against a Thousand Sons invading force. The GotC with the help of the trainees forced the CSM from the world and returned to the GotC ship.
Once they returned to the ship conversations began about how the trainees could be brought into the Chapter, and other conversations happened, regarding the purity of the Inquisition, or the lack thereof, and somewhere in the Inner Circle of the GotC it was decided to request the creation of a new chapter using GotC gene seed, the strike force that found the trainees and the trainees themselves. Using the world they'd liberated from the Thousand Sons as a home world, the Chorus Lucia was born.
Since then, the Chorus has ceased to think of its self as truly "Unforgiven" though they still refer to themselves as such. The thought started among the Inquisition trainees, the leaders of whom were in the beginning phases of Grey Knights training - that if the Chorus continues to remain as pure as possible, forsaking Chaos tainted tools, Xenos weapons, and even being wary of the Omnisiah, they would gain the Emperor's forgiveness for the crimes of their fore-bearers. The Inner Circle of the Chorus refers to its self, among its self, as the Forgiven. The Chorus still hunts the Fallen, and are quite successful in the task, but theirs is a hunt now born of righteous rage at the sins of the Fallen, then the guilt of a brother for his brother's sins.
I joke that I won't build a Techmarine for this army - the Chorus doesn't trust their Techmarines to leave the ships. What I might do eventually is build one of those Joquaro weaponsmiths the Grey Knights use, but stat him as a Techmarine, for further fluff. As it is I have a Legion of the Damned marine with a multi-melta that I'm going to work into either a command squad or elsewhere, and might eventually build a squad of Legion figures as a command squad, and in story say that the squad attached its self to the Chorus for reasons even the Chorus isn't entirely sure of.