#3 Brock Lesnar
The Beast Incarnate hasn't been seen since he attacked both Braun Strowman and Roman Reigns at Hell in a Cell and it was then declared that he would face both men in a triple threat match at Crown Jewel for the Universal Championship.
Lesnar costs WWE a lot of money every time he makes an appearance, which is why Paul Heyman is used in place of The Beast on Monday Night Raw most of the time, so it's highly unlikely that WWE would spend that much money on Lesnar just so that he can make an appearance in a small segment.
Lesnar, like many stars on this list, spent a lot of his career on SmackDown Live and the company has teased that he could be part of the show, but he could have priced himself out of the appearance since WWE has many other talents to bring in who are a fraction of the price.