Some quick answers:
"Will the expansion be standalone? Or will we have to own black plague?"
You will have to own BP.
"Will the expansion continue where black plague left off?"
Perhaps
"How long (as compared to Black Plague) is the expansion going to be? Several hours shorter, or just slightly?"
It will be around half of BP.
"You guys are planning on using a new level design, right?!"
Time will tell
"Will the expansion be geared more to survival or action?"
Puzzles actually!
"Any new technologies we'll see? Such as black plagues ripple effect?"
Same engine as BP. Nothing new planned.
"Will this be an end to all penumbra?"
That is one of life's great questions
"why an expansion pack?"
Because we felt it fitted with the ideas and length of Requiem.