He's actually probably one of the easiest to bring back. We're talking a time jump here of about 20 years. I think Giles can pass as a 20-year old. That's one thing I've thought about with so much of the main cast leaving. If they stayed, they'd have to age them 20 years. Rumple, being immortal, doesn't age so he's fine and Regina can keep herself looking young with her magic. I'm not sure what they're going to do with Hook, but Snow, Charming, and Belle would all have to be made up to be 20 years older every episode.