The following should turn face:
The Wyatt Family - They get a face reaction anyway, they should just make it official before much longer. They need more babyfaces with Bryan and Reigns out. We could have a great feud on our hands with The Wyatts and The Authority. 6 man tags of Wyatt, Rowan & Harper vs. Rollins, Orton & Kane could main event quite a few Raws. Eventually Wyatt would go solo and Rowan & Harper would turn on him. Wyatt vs. Harper could be a good feud as well, possibly even a Mania match. I think what they're going to do instead though is turn Orton face and put him against Rollins at Mania. You can kind of see the groundwork being laid where Orton has been tired of cleaning up Rollins' messes.
Cesaro - He and Wyatt were just on fire during the road to Mania and the couple months directly after. Then WWE basically pulled the carpet out from under them. While Cesaro and Heyman together looked great on paper, it just fell flat. The night after Mania is where Cesaro should've turned face. I'll never get why WWE wants to fight against natural crowd reactions for certain guys. If they had just gone with it, the sky would've been the limit for him. He may have even been the guy to take Bryan's place in the main event once he got injured.
Ryback - I really think there is still a glimmer of hope left for Ryback's career once he returns from injury. Hopefully they won't stick him with Axel upon his return to the ring. Rybaxel was going nowhere, so put him back into singles competition. There's nothing to lose by turning him face again and seeing where it goes. I would seriously consider giving him the US Title and putting him in a feud with Rusev, culminating in a match at Mania where Rusev would go over.
The following should turn heel:
Sheamus - He is in need of a character change in the worst way. He's one of the most stale wrestlers in WWE and has been for a while. I'm surprised that he gets any reaction at all, regardless of whether it might be from young children or not. We know from experience that he makes a much better heel, and there some new faces in WWE for him to feud with since the last time he was a heel.
Kofi Kingston - Another one of the most stale guys on the roster, if not THE most stale. Kofi has all the talent in the world in the ring, but zero personality. He's just one of the misused wrestlers in all of wrestling, not just WWE. I was very interested and hopeful when I saw what seemed like a heel stable form between him, Big E, and Xavier Woods a couple months ago. Of course it led nowhere, and Kofi continues to job. Kofi and Big E could form a heel tag team (the division needs more talent, but that's a whole other story) or Kofi by himself could make a great opponent for Ziggler.
Big E - See above.
Mark Henry - The wheels may have already been put into motion for this, with Henry showing a more aggressive side towards Bo Dallas. Henry plays an awesome heel character, so I hope this is where it's heading. He could put on a good feud with Roman Reigns, putting Reigns over on his way to a probable championship win at WM31. If Cesaro were to go face, then he and Henry would be another program I wouldn't mind seeing.