#4 John Cena
John Cena has been very public about the fact that he won't be able to devote as much time to WWE, as he once could. Therefore, the company may want to make good use of him, when he is around.
John Cena may be dwarfed by Brock Lesnar physically (who isn't?), but remember that he's one of the strongest men on the entire roster. Imagine the potential of him trading insults with someone like Paul Heyman to build this feud up. We've seen this match in the past, but we'd seen Goldberg vs. Lesnar too, and that was a pretty cool series indeed.
John Cena vs. Brock Lesnar circa 2017 is a clash for the ages, between two men who came up the same way, from pretty much the same era, and are now synonymous with sports entertainment. Also, remember that John Cena brings out the best in every single performer.
Here's another repeat clash with immense potential to be quite massive indeed.