#4. Rey Mysterio - An opponent Roman Reigns has a small history with

One of the earliest signs of WWE's intent to push Roman Reigns happened at Survivor Series 2013. Just a year into his WWE debut with The Shield, the trio teamed up with Jack Swagger & Cesaro (The Real Americans) to take on The Usos, Goldust, Cody Rhodes, and Rey Mysterio.
While the babyface team had the advantage in the early going, it was Roman Reigns who was the biggest difference-maker, eliminating four out of the five possible opponents. Seth Rollins, meanwhile, scored an elimination against Jey Uso before getting eliminated a few minutes later by Rey Mysterio.
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Roman Reigns was the sole survivor and eliminated Rey Mysterio at the end to win the match. They haven't had a serious singles feud, and given that both men are on SmackDown, we wouldn't be surprised to see Roman Reigns eventually get a victory over the WWE legend Rey Mysterio.
It would likely be a one-sided beatdown from Roman Reigns.