The scenario you described is already possible now, if you're worried about deliberate breaches of privacy. If your concern is about accidental breaches of privacy, there are ways to make that harder, eg by having a checkbox that enables the extra recipients.

The breach of privacy I was thinking of was more along the lines of you learning (and maybe passing on to others) that in addition to you, Sophie sent her message to Alice and Carol. That information isn't available to you now, and the change wouldn't make it possible.