Long term, the obvious issue is replacing Matic. Every other player in our current best XI have years of prime football ahead, injuries allowing.
I have to say I'm fascinated by attacking fullbacks, and since AWB is so good defensively I'd love to see the world's best attacking LB bombing down the left. We need Rashford to move inside the box into striker positions anyway, his success as a wide forward has all come playing counter attacking football and relying on speed. Or we need to find a way to counter attack in some games to take advantage of him.
After that I would like to see a right forward/winger that can dribble past people and has the vision to play balls into packed defenders or over them.
Looks like Henderson can take over for De Gea if he doesn't shape up.