#1 Julian Weigl (Borussia Dortmund)

The top player in this list, for a few reasons, is Borussia Dortmund's Julian Weigl. Someone blessed with world-class potential, he'd represent an excellent signing and one that would succeed Sergio Busquets as Barca's midfield metronome in future.
His presence would benefit Barcelona, especially in the long-term. Pep Guardiola, while at Bayern, had previously been interested in him. He has since reignited his interest, according to reports. It's not hard to see why.
At 22, Weigl epitomises a refreshing change from the traditional midfield player. He's booked more than he scores or creates assists, but does precisely the job which allows his attacking teammates to flurry forward with regularity.
Weigl occupies the role between defence and attack in midfield, doing so with both poise and precision to impressive effect. In 26 games across both the Bundesliga and Champions League this past season, Weigl averaged 87% pass accuracy, 45 completed tackles and 37 interceptions too.
Even just by looking at those statistics, it's frustrating that he did not play more regularly this season. Injuries have played their part though, as he has suffered from troublesome knocks and setbacks in recent seasons.
A muscular injury last month has stopped him from featuring as part of Germany's World Cup squad, an untimely blow for such an integral player.
Stats: Squawka