#4 WWE Universal Champion Roman Reigns
Since Roman Reigns made his return to SummerSlam in 2020, he has proved that he didn't miss a beat. Within a week, he conquered The Fiend and Braun Strowman to become the Universal Champion. Currently, Reigns has been Universal Champion for eight months during which he overcame every challenge.
Throughout his career in the WWE, Reigns has defeated WWE's top superstars, including John Cena himself. The last time these two top stars went against each other was at No Mercy in 2017, where Reigns cleanly defeated Cena, thus earning his respect.
Cena's retirement doesn't have to be for the WWE Championship specifically; it can also be a championship Cena has never competed for.
Cena has never held the Universal Championship, which is why it would be interesting to see Cena challenge "The Tribal Chief" for his possession. A loss to Roman in Cena's retirement match would be something the WWE Universe would be talking about for a long time.