#2 Big E cashes in on Roman Reigns ending his reign
Let's assume Roman Reigns defeats John Cena. As he celebrates in the middle of the ring, Big E's music hits as he cashes in on The Tribal Chief. It could start a feud that lasts all the way until Survivor Series 2021, as Reigns plots and ploys to get his Championship back!
It makes more sense for Big E to cash in on Roman Reigns once he's exhausted after the match of his life, because it just makes things more personal, going forward. Imagine the immense heat!
#1 Roman Reigns defeats John Cena clean
John Cena has come back to WWE to give the active roster the rub. Riddle even spoke about getting the rub from John Cena and Randy Orton's reaction to the same in the video that's been linked right above.
This is why the Roman Reigns vs. John Cena match could conclude with the latter putting the WWE Universal Champion over by suffering a loss in the middle of the ring. It would give Roman Reigns bragging rights for many months!