I'm hoping to see Leaf & Red in battle. As I said, Leaf could make Kanto's White Rock more important than Sinnoh's, which has importance for a Nintendo Event, by being battled there. Red has to stay in Mt. Silver, if only for nostalgia.
It'd be great to see Red and/or Leaf appear in contests after you beat them. Maybe other characters will appear in the audience, like Jasmine did in R/S/E.
Leaf as a coordinator makes sense. And Daisy would make an appearance as one (since she is one). Bring this on. Let Red stay where he is.To see them in contests would be a great idea. Though I really wouldn't want to see Red as a coordinator. Maybe he could be the Elite Champion while Leaf shows up in contests.