I like how the sourcebooks don't have to portray the stereotypes, and I can easily see the monodominant book being an interestig prospect. I think Marco's idea about the monodominant using humans with useful mutations to smite his enemies, but think that he would treat them as sub-human, and maybe use them simply because if they get killed, it's one less living mutant in the imperium.
Sub-factions within the monodominant faction could be interestig, with some who won't tolerate mutants through to those who believe that while it is humanity's right to rulethe stars, humanity always evolves, and mutants are the next step in the evolution of the human race, and therefore more worthy successors.
The Xanthite sourcebook would be a great place to introduce some more 'exotic' wargear, such as an Anathame (anyone read false gods?), and also, imperial technology which is regarded as heretical. One needn't use /chaos weapons to cross the line...
EDIT: I would also support the background of Marco's namesake appearing in the next dark magenta, as that way, we could slowly get people more familiar with most of the characters they would see at conclaves. We've had Sassure and Kaled, so why not Marco. Plus, it would make a good few pages of reading for the magazine.