#2 Braun Strowman

These two men are so over that seeing them clash just makes sense. It will be a true clash of the titans when these two collide. Strowman dispatched of Cena pretty easily in the Elimination Chamber and that could be incorporated into the feud, playing up how much of an underdog Cena would be against the Monster among Men. Having wrestlers and announcers giving Cena no shot to slay the monster could put him in a place we haven't seen. He himself may not believe he can beat Strowman.
Strowman could absolutely destroy Cena and that could force him to start thinking of retirement. During a leave of absence for Cena, Strowman becomes Universal Champion. This would motivate John to come back to face Srowman again; win, lose, or draw he would give the monster his best shot.
Cody has no chance against John Cena? Details HERE
A desperate Cena trying to scratch and claw his way past the most unstoppable force WWE has ever seen, sounds like a great feud. Passion is Cena's strong suit and seeing him on the brink of retirement may just give us the peak of his performance.