#4 Booby Roode
During his time in NXT, Bobby Roode played the role of the narcissistic, aggressive heel, which helped put him on notice as one of the best talents on NXT. Roode also had considerable success on Impact Wrestling playing a similar role and as one half of Beer Money Incorporated. Its clearly a role Roode knows and given his somewhat less than auspicious start to life on the main roster, certainly a route WWE should consider going down.
Right now when you think of Roode, you really just think of his entrance, but he clearly has a lot to offer in WWE in terms of his character and wrestling ability. A heel turn could reinvigorate his character and push him in the right direction similar to where he was in NXT. If he were to emulate the role of the narcissistic, cocky heel he played in NXT, then WWE have a top-level heel capable of holding the Universal championship and feuding with Roman Reigns.
Right now Roode is nothing more than a mid-card level talent who is destined to stay there unless his character is transformed. Changing his entrance music, although it would be unpopular amongst fans, would be a serious step towards vilifying him and pushing him as a credible bad guy, as would having him cut 'holier than thou' type promos each week on Raw.