#2. Daniel Bryan
A rivalry with the recently-turned heel in the WWE Champion himself, Daniel Bryan could also be best for business.
Bryan & Cena have gone face to face in a high-profile match before at Summerslam 2013, when they went one on one for the WWE Championship.
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This was during the midst of Bryan's epic 'Yes Movement' storyline, and his rivalry with Cena had culminated in that one match as Bryan knocked off Cena to become the WWE Championship- shortly before Randy Orton cashed in his Money in the Bank briefcase to rob Bryan out of the title following some help from Triple H.
However, we haven't seen the two men engage in a match ever since then. 2019 would be the best possible timing for Bryan & Cena to go at it once again, especially now that the roles have reversed. Bryan is now playing the role of a heel, whereas Cena is still a babyface.
There are so many directions that WWE could take this storyline. Let's also not forget that the fact that Cena has actually never defeated Bryan in a match, so this could also be the catalyst for this story. A rekindled rivalry with Bryan playing a more cold-hearted role could provide a very personal touch to the rivalry that we didn't see much of from their past encounter.
John Cena would be the perfect option to question Bryan's motives into turning into such a despicable person, and as we've seen before- the WWE Universe would be up on their feet for another bout between the two popular stars.