Anything facts about episodes not yet aired are spoilers. Please use spoiler tags. This includes press releases as well as things you've found out. Basically if it hasn't been shown in a Doctor Who episode on TV then it'll go in spoiler tags.
[NOPARSE]
Spoiler
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Speculation is fine and doesn't need to be put in spoiler tags unless it's based off info which would go in a spoiler, in which case spoil your speculation as well
Please spoiler things like this - I don't mind reading them, but some people do not want to find things like this out. As a general rule - if it's details on an episode yet to air, keep them in spoiler tags.