Missy doesn't seem to have been doing much throughout the series, in the Forest being a Chorus like the ghosts from Stardust. The Robots trying to reach the Promised Land seems the closest to this arc actually affecting the stories. But the trailer looks exciting. Cybermen and UNIT. The Doctor in what looks like Hell. And Missy says the Doctor knows who she is. Who is this Mary Psychopompins?
Ideas are the Master, Rani (one of her experiments and they could say in memory of Kate O'Mara), Valeyard (a being like the Watcher), River Song (in the Library?), someone else in the Library, Romana, Susan's Grandmother, a relative, another Time Lady (agent of Rassilon), Tasha Lem or someone connected to the Papal Mainframe (they can apparently notify the afterlife), a version of the TARDIS (gone evil like in Zagreus), another companion, something from the Matrix (personified thoughts), a being similar to the Hypothetical Gentleman of IDW, a version of Clara, a construction of the Cyberiad, the Moment, Death, Time, a Maia of Mandos, a Maia of the Guardians, a rebellious Maia like Sauron (rebellious Disciple of Light?), one of the Guardians (Gold is Life and Death), an Eternal, a Chronvore, a being from another dimension like the Boneless, something connected to Gus the Computer, G.I, some other Eldritch Abomination like the Animus or Fenric taking a form you are comfortable with, something like the Beast. The possible Hell in the trailer may be connected to that. For more ideas on Series 8 go to TV Tropes WMG page. Or is Seb the real villain and one of these? Will previous dead characters appear? Did a previous villain die and try taking over the Nethersphere, letting the Cybermen in? Is Seb the Half-Face man? Will Davros appear? Will anybody leave the Nethersphere? And she says the Doctor knows her but has he met her yet?
Anyway, I look forward to Dark Water. Which reminds me of the Game of Thrones episode Blackwater that beat Doctor Who for an award last year. I suspect this story will be submitted.