I think Dortmund should make a new start.
Sell Gündogan, Ginter, Reus, Weidenfeller and Immobile and maybe some others, use the money to get back Bittencourt, and buy Firmino or De Bruyne (even if they are expensive) and Werner from Stuttgart, Horn or Zieler as gk. Build a new team around young players from Germany, Brazil and Argentina, avoid release clauses and build this thing new, better and longer lasting. I think this will work out on a long term, they wont play CL next year anyway so this might be the right moment to get rid of everyone who wants to go or who's constantly injured or passed their prime for as long as they have a price tag above their name. Keep Aubameyang because the guy works his áss off there and is a blessing.
What Dortmund is doing at the moment is nothing else then waiting, they seem completely stuck in their hopes of all the off-form players finally getting back to their old best (Gündogan, Kagawa, Sahin) and for the recent signings to finally explode, but I think this will be a waste of time and in the meantime their other best players leave for free or for an insanely low fee that cant buy proper replacements. Renew this team as long as it is interesting for talents. Its risky, but it is better then waiting for your own grave. Dortmund did it once and they can learn from the mistakes they did in that time. They can do it better in a 2nd attempt. Most BVB fans would slap me for this post but I think they should finally start thinking 2 steps ahead, instead of drowning in self-pity, and the delight of yesterday's success.
The faster Dortmund recognizes that they are stagnating and forced to do a move, the higher the chances stand for a decent restart.