So I went ahead and actually compiled this list of Champions, rivals, and plot-important characters and organized them by gen. I didn't list the rival or friend of gen 6 because I'd much rather get these characters than them. And I didn't include protagonists because as cool as it would be for them to return, if space really is limited (I don't want to start getting too hopeful too early) I'd rather have established characters return than silent protagonists.
Red, Blue
Lance, Silver, Eusine
Steven, Wallace (New Champion outfit?!), Wally, Zinnia
Cynthia, Barry, Looker
Alder, Iris, Cheren, Bianca, Hugh, N, Colress
Diantha, Sycamore (he would truly be the ultimate threat), Emma
I also wanted to add characters like Benga and the
gen 4 stat Trainers, among numerous others, but I'm worried that would start pushing it...