Everyone has outright said Culber is coming back, and it will piss off a loud section of the audience if he doesn't. He will return through some sort of Mycelial Magic and I don't think it will be a long wait.
Georgiou's exit was blatantly set up in such a way as to allow her to come back as a recurring character, once or many times. If she doesn't, it's because The Powers That Be couldn't come to an agreement with Michelle Yeoh. And I doubt that, since she was obviously having a grand time chewing scenery (and ganglia). She could be the Q of Discovery and I for one would be down for that.
I am fairly confident that we will also see Prime Lorca, but I'm not sure when or in what capacity. This may have been part of the plan all along, or not. If not, I'm sure shit is happening to make it so. Jason Isaacs was not only stellar on screen, he did a better job of promoting Discovery than the whole damn marketing department, and if he didn't love it he's an even better actor than I thought. I could believe anything from a one-shot deal where Lorca resurfaces (or is rescued) to him showing up as captain in Season 2 Episode 1. I do NOT think Mirror Lorca is coming back. I think he's pretty thoroughly dead. Except MAYBE as a malevolent ghost haunting Stamets inside the network. I could envision that.