#4 John Cena
It's easy to forget that John Cena and Braun Strowman did cross paths a couple of years ago on RAW. There was no conclusive finish, which was likely to protect both superstars and save the actual match for a bigger occasion.
We're not sure who's going to win the Firefly Fun House match between Cena and 'The Fiend' Bray Wyatt, but we expect the former Universal Champion to win. Even if Cena loses, that doesn't mean he can't challenge for the Universal Championship.
He's gotten World title shots just for "being John Cena" and he doesn't even have to beat Wyatt to challenge for the Universal Championship. It's not a feud that needs to be Strowman's first title defense, as Cena will mostly not stick around after WrestleMania 36.
But a feud against Cena would be a great one and there's no doubt that Cena would do his best to help put Strowman over as Champion. If anything, his squash match against Brock Lesnar was an example of his willingness to put someone over big.