#4 Lars Sullivan
This was a match that was rumored for last year's WrestleMania, but nothing came to fruition. It's said things were changed because of Sullivan's mental health issues. Then, he suffered a knee injury that put him out for 6 - 9 months.
A recent report suggested that he could still be out till June 2020. But it all depends on how fast he recuperates. This is all assuming that he can get his knee sorted by the time WrestleMania 36 rolls around.
WWE had plans to make him the Big Monster heel and there's no better way to do that than to have him destroy Cena.
The company tried to do that when he destroyed Kurt Angle on an episode of Monday Night Raw earlier this year. It would only make sense for them to continue with this 'Destroyer of Legends' gimmick and give him an opponent that would put him over with the rest of the fans.
Cena has been more than willing to do what the company asks him to, and putting someone like Sullivan over could just be in his capacity.