#4 Dean Ambrose: The Lunatic Fringe

In 2014, Seth Rollins betrayed not only Roman Reigns but also his unhinged stablemate, Dean Ambrose. The Architect and Ambrose had a great feud after the breakup of the Hounds of Justice and finally reconciled at Summerslam 2017, sowing the seeds for a Shield Reunion later in the year.
While he's found decent success in the role, Ambrose has played the role of a babyface for far too long. A heel turn and subsequent feud with Rollins after his return from injury would allow the Lunatic Fringe to revitalise his character.
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After all, Ambrose has all the attributes needed to play a great heel and will almost certainly produce more compelling television with Rollins.