Seth Rollins, Kevin Owens, and Dean Ambrose in that order.
Rollins is so talented, and already has the main eventer credibility.
Owens is just undeniably. That guy just has "it". He's too good to not be up there. I hope they really make him important at Wrestlemania.
Ambrose has the history with Reigns and Rollins and the fan reaction. I think he will be an amazing heel when they eventually let him turn too.
Also the eventual Shield reunion when all of these guys are the top stars will be incredibly fun.