Saga of the Beast will undoubtedly be Space Wolves VS Orks, as Space Wolves love their Sagas
Which leaves Custodes, Death Guard, Harlequins, Inquisition and Necrons.
I've noticed that in the most recent White Dwarf there's an article of fluff concerning the possible return of Szarekh the Silent King (the Necrons' long-lost ruler), which will probably be the subject of the next Psychic Awakening supplement after Saga of the Beast and Machine War. It would be great if they did actually give him a miniature, and if they weren't going to bring him back, why would they dedicate the time to putting that article into White Dwarf?
Given that there are number of Inquisitor holomessages featured in that article, I wouldn't be surprised if the opposing side in that supplement was the Inquisition, as Inquisitors are sent to gather intel on this latest development and try to stop the Silent King before he unites the Necron race to eradicate all would-be usurpers!
That would then leave Custodes, Harlequins and Death Guard to enjoy a 3-way punch-up a-la The Greater Good, which might well give Harlequins a much-needed second release for some variety.