#4 Seth Rollins
Bringing Rey Mysterio back as a challenger for Seth Rollins' Intercontinental Championship would be a fantastic way to re-debut the ex-WWE World Heavyweight Champion, and it might even make the IC Title the hottest belt in the entire company for the duration of the feud.
Unlike with Balor, you probably couldn't turn Seth Rollins heel in this program, as the fans probably wouldn't want to boo him. Also, if you ask me, Rollins is a much better babyface than he is a heel, and it's pretty hard to imagine Rey Mysterio as a heel, so if WWE were to book this program, it'd be a babyface vs. babyface feud.
Of course, in the end I'd expect Seth Rollins to retain the Intercontinental Championship while having some of the best matches of his WWE career.