3. Shinsuke Nakamura
Cena and Nakamura have faced each other once on Smackdown Live to determine who would face Jinder Mahal of all people at SummerSlam for the WWE Championship. Nakamura won and Cena endorsed him properly for the victory.
But the roles have changed a lot since then. Cena has suffered an embarrassing defeat at the hands of the Undertaker while Nakamura has turned heel and won the United States Championship from Jeff Hardy.
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Although a Jeff Hardy - Nakamura rematch has been made official, Randy Orton seems more focused on Jeff Hardy at this point, than Jeff is on the US Title. So a Randy vs Jeff grudge match should be the best option given how personal Randy has been in his attacks.
That leaves Nakamura free at SummerSlam and in comes Cena. John Cena had been one of the greatest US Champions when he won the title at WrestleMania 31 from Rusev. His US Title Open Challenge was a thing of beauty, and Cena could benefit from winning the title again.
Given Cena will be a part-time act until Bumblebee launches in theatres, it will be unwise to hand him any title. So keeping the title on Nakamura would be logical while a win over Cena would make him look stronger.
Their Smackdown match hadn't been exactly a 5-star classic so we can expect both men to step up their game and deliver a better match than what they did before.