Moriarty is dead, and here comes another villain. The upcoming season is expected to explore darker themes, compared to the past seasons. It was speculated that Mary will eventually die in the series.”
Shouldn’t it be “would” not “will eventually die”?
Since you say "upcoming season", I assume that the outcome is not yet known (at the time of writing). In that case I would use "will", but consider whether "It is / has been speculated that Mary will eventually die in the series" would be better, especially since you have just said "is expected".
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Since you say "upcoming season", I assume that the outcome is not yet known (at the time of writing). In that case I would use "will", but consider whether "It is / has been speculated that Mary will eventually die in the series" would be better, especially since you have just said "is expected".
You are right.
Backshift happens quite generally in constructions where one clause is embedded within a larger one containing a past tense verb.
It is not certain that Mary will die in the series - the current speculation may be different - so stick with "would" and you won't go far wrong.