The thing that I liked about Gary Seven is that he was part of the 20th century, and I like stories set in our past. Into Darkness is not set in our past, so I don't see why I'd enjoy Gary Seven more than I'd enjoy John Harrison or Mary Williams. If the villain is Gary Seven I won't get any additional enjoyment because of it, and it will be barely more than just a name. OK, I'll get some references in my head and some tiny bit of excitement, but that's it.
I'm much more looking forward to seeing Carol Marcus.