#4 Shinsuke Nakamura

Although he's failed twice to win the WWE Championship from Jinder Mahal, we can't entirely rule out the possibility of Nakamura challenging for the Title a third time.
SmackDown Live desperately needs top babyfaces (and heels, to be honest) and Nakamura -- if booked right -- can certainly be one of the major players on the Blue brand.
How Brock Lesnar ruined another wrestler's career - Watch Now!
From what we hear, he's on main event level money so unless the WWE plans to let him go or terminate his contract, I don't see how he isn't in and around the main event scene.
Perhaps giving him another shot right away would be overkill, especially considering that he didn't have the hottest feud with Mahal, but I certainly wouldn't be averse to him getting another opportunity in the near future.
A top WWE star is missing in action