#1 The top guy
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It is no secret that Roman Reigns was the face of WWE. He was the centre piece of the company, and the entire storylines were built around him. Reigns main evented four consecutive WrestleManias - something even John Cena, Stone Cold Steve Austin, or The Rock didn't do. Hence when Roman stepped down, there is suddenly a huge void to be filled.
WWE could tackle this in two ways - either have a new top guy or employ more than one top guy. During the Attitude Era, there was more than one top guy in the company. However, lately it hasn't been the case - especially after 2010.
Top name doesn't want to enter the WWE Hall of Fame. More details HERE.
Now WWE has Braun Strowman, Drew McIntyre, and others to contend to be the top guy of the company. It is entirely possible that WWE may go back to having more than a top guy to keep the main event scene fresh. Once Reigns returns, he can have feuds with multiple top stars.