The league is over. Dortmund, who are back in the top 4 with their recent surge and a Champions League ticket, can hopefully be in the title race next season. Their star players Guirassy, Brandt, Gross and Schlotterbeck all finished well. They collected 22 points in the last 8 weeks (7 wins, 1 draw). I wish the Nuri Şahin era had ended earlier and Kovac had come earlier. I'm sure they will do well next year. The team needs some good transfers, at least 2 to the starting eleven. This is necessary to compete with Bayern München and Bayer Leverkusen.
Dortmund doing well next season because they finished this season strongly and got into top 4 and Champions League. Getting 22 points in last 8 weeks is really good. You mentioned that some important players like Guirassy.. Brandt.. Gross and Schlotterbeck played well at end of season which is good sign for team future. Nuri Şahin had started earlier and Kovac had come sooner team might have done even better. They played well under new coach.
Now looking forward you are right it is very important for Dortmund to buy some good new players. This is important if they want to have real chance of competing with Bayern München and Bayer Leverkusen for league title next season. Getting those good new players could give them extra strength they need to try and win.