Being able to mute unknown senders is great, but I'd miss GrubHub and Amazon arrivals! Would love to be able to set a custom notification sound for All Unknown senders (or All Known senders, with the default applying to Unknown). "All Existing" might also be helpful, for messages that go to existing conversations! Ooo, ooo, and maybe "All Services", covering any message from a non-10-digit phone number. But especially All Unknown :D