#3 By interjecting himself into the Roman Reigns & King Corbin rivalry

If you take the Universal Championship rivalry involving ‘The Fiend’ Bray Wyatt, Daniel Bryan, and The Miz away from SmackDown, the only consistent rivalry left is between Roman Reigns and King Baron Corbin. The two Superstars have been at it for several weeks now and traded victories in the process. They have also attacked each other during and after matches to keep the heat on their feud.
Sheamus is a former World Heavyweight Champion, and WWE will be looking to bring him back in an effort to build him back up to prominence. While SmackDown is continuing the Roman Reigns and Baron Corbin feud, we could see Sheamus appear and attack both men in order to get himself into one of the Blue Brand's hottest rivalries. This would allow him to compete with both men simultaneously and keep the option open for a singles rivalry between he and either Reigns or Corbin at a later date.
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At a time when WWE will be looking to take Reigns and Corbin’s rivalry into the Royal Rumble, we could see 'The Celtic Warrior' insert himself and then carry on the feud with one of the two men heading into a grand match at WrestleMania.