#4 Samoa Joe
The Samoan Submission Machine is currently out of action with a knee injury and is also "The Right Hand of Destruction" for Triple H, but there is someone else who was once close to The Authority and it seems that this partnership isn't destined to last very long.
Seth Rollins should be a cautionary tale to Samoa Joe that when The Authority decide that you are past your sell-by date, they move on. Rollins was once in Joe's position and he suffered a knee injury too; this was the injury that saw the downfall of "The Future of WWE" until he realised that he could slay the king.
Joe has experience, he has the technical ability and even though he is a heel superstar, he could bring something different to The Shield to help them when they are facing much bigger opponents.
Joe would be the addition to The Shield that would make them unstoppable regardless of which team attempted to stand in their way.