The Promised Land was a common reference in the first half of Series 8; it was the goal of both the Half-Face Man in Deep Breath and the Robots from Robots In Sherwood. Now this could easily be taken as proof that it is a real location, but that ignores that the chief link between the two villains is that they're automated. Could a human belief in heaven have manifested as a tangible place, when in fact it's more abstract? OK, probably not. What we do know is that the Promised Land/Nethersphere is central to the finale and the series' overall arc as it's been explicitly stated where Missy and Chris Addison's Seb appear to rule over. What purpose it serves, however, is unclear. It's where people go after death and appears to be unselective (it's not just people Missy wants to talk to). On top of that, it moves forward in time with the Doctor, rather than the rest of the world (we see people from Victorian London, the future and present turn up in that order). Conclusion: We know Missy is a tangible entity, meaning that the Promised Land must also exist in some form too. It seems like, instead of being a literal heaven, there's some spiritual stuff going on that's bringing recently dead people back into the real world. Or maybe Missy's God.
