What I envisioned for the Necrons is pretty much as-per canon, but to a point...
Szarekh the Silent King returns from his self-imposed exile, after dealing with a force of Tyranids with the help of a loyal legion (not the Blood Angels, because they're bad guys in this story). He will meet the Emperor, but I'm unsure right now what will be said or what they will think about each other. As-per canon, Szarekh will view the Tyranids as the greater threat, for if they devour all organic life, the Necrons will have no way to reverse the biotransference process. So on occasion, he will "help" the Imperium and/or Tau destroy the Tyranids attacking them.
The Silent King will not be without his own enemies though. Chief amongst them will be Imotekh the Stormlord, who seeks to remove Szarekh from the throne and take power for himself.
EDIT: I just remembered something about the Sons of Horus. I think in the previous topic, it was decided, or at least discussed, that the 16th legion would remain under the name of Luna Wolves. The three remaining Mournival members; Aximand, Torgaddon, and Garviel Loken, would lead it as a triumbrant of sorts. They'd also retain their off-white and black legion scheme. Only Abaddon would turn traitor, as it has already been listed so.