You are right, Leverkusen was unbeaten last season. Even Munich could not catch up with a fairly large point difference. But Dortmund did not perform well, it was not better than Leipzig and Stuttgart. This season the competition is also tighter. Leverkusen is no longer unbeaten like last season. Some teams are also getting stronger, if Dortmund can not keep up to improve their game, they will be further displaced.
To be honest in my opinion this season is more interesting for now as there are more teams that can make it to get the title.
I already said and written it a few times , the first 6 Teams still have a chance for it as the point difference are not that much.
I hope that Dortmund will be making some improvements for the second half of the season.
What also would be awesome that maybe Frankfurt getting the title.