It was good. Not a spectacular episode, but it was good. Bill is growing on me. She seems to have a more natural response to traveling with the Doctor: equal parts wonder and terror. Some of the recent companions have seemed too eager from the beginning to be quite believable to me. Sad she is leaving at the end of the season as well.
(Rumor is, Capaldi didn't actually want to leave, but jumped before he was pushed. Apparently the BBC has decided the low viewership of recent years is because Capaldi is too old to play the Doctor. (And nothing to do with them scheduling it in a "kill slot"). So they've decided that with a new show runner coming on they need a new Doctor, one who is younger and sexy and male. Someone like David Tennant. And a new companion as well. All rumors do far, but it does jive with how the BBC has treated the show in the past. Sorry to sound elitist, but if your only complaint against Capaldi is that he is too old, you aren't a real Doctor Who fan...)
As for what's in the vault...no idea. Missy kind of seems like a safe bet. Gallifrey is possible, weird, but possible. Could also be the Doctor's next incarnation for whatever reason. (As for the Valeyard, well, anything is possible, but I'd say unlikely: from what we know it seems much too late in the Doctor's life for that. Chronologically speaking, the Valeyard would fit in sometime between Tennent2 and Smith, so during the year of the specials (The Planet of the Dead, The Next Doctor, The Waters of Mars, The End of Time").)
Anyone else sign up for BritBox? They've finally added Classic Who. Mostly complete: besides the missing episodes there are a few stories unavailable at the moment (Remembrance of the Daleks for instance.) Also, unlike when they were on Hulu, BritBox isn't streaming them in HD. Actually, everything I've watched on there (not just Doctor Who, but everything) looks like bad quality VHS. I know it's not on my side (can have 3 to 4 HD streams on other sites running simultaneously and still look great). So has to be on their end. :/