Hummels captain key player
Gundogan VC important player
Reus awesome player
these 3 are your key players
Subotic is a very good CB along with the Greek Papastathapoulos and you have a great upcoming CB in Ginter. Give him a run in easy games and let him develop. Do the same with the young RB Passlack..
There are 2 very decent LB's in the team.. Schmelzer and the other one I forgot name...
Kagawa is also very good, but you can replace him in sometime..
look at bringing Gabriel Barbosa asap if you can..
add in a solid new RB- Mario Fernandes would be my choice..
Aubameyang as F9 or right IF is a very good player...
Grosskreutz is very versatile, and that imo is his failure! in the sense that you can't identify a correct position for him... though he would be my first choice as a sub as he covers just about every wide position and can play certain central positions also.
the first position you might look to strengthen is the GK department... coz if you are looking to dethrone Bayern and compete in Europe, Weidenfeller ain't the best... Timo Horn has a low release clause and turns out to be great.. Jeronimo Rulli is awesome and can be a perfect choice from the go..
Lucas Romero or Gaston Gil Romero are good options you can consider to add to the midfield...
* I just recalled the Dortmund team and players from my memory amd might be wrong with some or their spellings... also, I guess there's a very good striker in their ranks.. immobile?