#2 The Rock

The big rumour before Roman Reigns' sad illness was that his cousin and former WWE Champion, The Rock, would return at Royal Rumble, win it, and set up a match against Reigns at WrestleMania 35.
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We aren't sure if The Rock is still in the WWE's plans at the moment, and if he is, the WWE may have few feuds to engage him in.
The Rock has been away from the WWE for three years with his last match coming at WrestleMania 32, while his last appearance at the Royal Rumble PPV was way back in 2013, when he faced CM Punk for the WWE Championship.
The Rock's Royal Rumble appearance will get a huge pop, as well as coverage from the entertainment and mainstream media, which the WWE wouldn't mind, but I think it would serve no purpose, and WWE can instead hold him back for a match at WrestleMania 35.