#3 Samoa Joe
From the moment Samoa Joe enters a building, you know he means business. As the crowd begin the JOE chants, we know we have a potential brawl.
However, SummerSlam wouldn't witness the Samoan Submission Machine put anybody to sleep. And that is a loss.
Joe has had a start-stop time off late. Every time we think that he is finally getting a push, something goes wrong. His feud with Rey Mysterio could never reach top gear, largely due to injuries to the Master of 619.
Then, he dropped his US Championship to Ricochet. And, most recently, Roman Reigns beat him clean on an episode of RAW. From there on, he's just been a supporting actor in a larger Roman Reigns story.
Perhaps WWE could have waited to start Roman Reigns' attack saga, and had him face Joe at SummerSlam. It'd have been a classic bout.
Another monster, who comes next in this list, is also set to remain absent from the card. If Reigns vs. Joe wasn't possible, WWE could have had something remarkable there for Joe leading up to SummerSlam.