2.Would make the attack more fluid
If there was a common criticism of Manchester United last season, it was that they were far too predictable. It looked as though their aim was to simply get the ball out wide and put in cross after cross, without worrying about whether there was anyone inside or not. None of the wingers interchanged positions with any real effect and that made them easy to play against.
The addition of Reus would make the attack immediately more fluid. A front three of Mata, Reus and Januzaj would make the attack highly fluid. All three can interchange positions with great effect and can also be effective there. Mata started his career on the wings at Valencia, Januzaj can play on both wings and Reus can play centrally and also out wide. This would make the attack more dynamic and a lot more difficult to contain.