User blog comment:The Time Lord/Doctor Who - Series 12 Premiere Hints/@comment-27270572-20151009222311/@comment-15395126-20151011065312

With Series 9 airing, my inspiration is back, so I plan to :)

Now for the true meaning(s) behind the above hints…
 * "The bitch is back." - Yes, Missy is going to return in the first episode of Series 12, assuming something isn't done to prevent this.
 * "An unconventional TARDIS" - The Valeyard, the main villain of Series 12, is using the Doctor's TARDIS to accomplish his goal. Getting it back from him is one of the goals of the episode. However, to time travel, the Doctor needs to use Missy's TARDIS. I envisioned it being a black and gold sports car :P The sight of that in the 17th century makes it "unconventional," not to mention its shape is very unlike that of the Doctor's TARDIS.
 * Louis XIII - This was to place the episode in terms of time period more than to suggest a character's appearance; while Louis may make a cameo, at most, this is meant to suggest 17th Century France as the setting for this episode.
 * "There is some evil in all of us, Doctor. Even you." - This is actually a quote from "Trial of a Time Lord" spoken by the Master. It hints at the Valeyard's return, as well as the Master's/Missy's involvement in the episode/story arc.
 * "Gateway to Gallifrey" - This could be the device that connects the three story arcs of the Thirteenth Doctor's era together. See your talk page for the latest on this ;)

Overall, nice job on the guessing! :D