Well, Balor is on RAW so that wouldn't really sound realistic.
Joe is on RAW too, so that wouldn't really sound realistic either.. Unless he just was specifically hired by HHH as his hired gun to try and take out Seth Rollins.
Nakamura is currently on NXT with an impending rematch...
So either Cena, Orton, or Joe would be the only realistic option for Smackdown's main event match
Cena/Styles for the title
Orton/Styles for the title
Joe/Styles
They should bring back that RAW vs Smackdown matches at Wrestlemania. Styles vs Balor would be a sight to see on the big stage