Yeah, that hope has been around since the day DND was introduced, along with being able to customize DND based on day of the week (weekend vs. weekdays, for example). And yet here we are. The closest thing that was recently added in iOS 10 was emergency bypass that would allow messages (or calls) to come through, but that not only bypasses DND it also bypasses the silent switch settings, so if you have your phone on silent/vibrate it will still actually ring too.