What could the "darkness" be? Perhaps the star desert surrounding Gothos?
Nah. Probably something new to this movie.
Honestly, I think we try too hard sometimes to connect every tidbit of news about this movie to some obscure bit of TOS trivia, which isn't really how the movies have ever worked.
Think about it: Khan was really the only time the Trek movies ever referenced a specific old episode in a big way. Most of the time the films were about new characters and threats: V'Gr, The Genesis Device, Carol and David Marcus, Saavik, the whale probe, Gillian Armstrong, etc.
So I'm always skeptical regarding theories that heavily involve Khan, Gary Mitchell, Gary Seven, Trelane, or whomever . . . .
As I've joked before: if this board existed back back in 1978 or so, people would be insisting that Persis Khambatta had
to be playing Elaan of Troyius!