That is very good way to look at next season in Bundesliga. It is likely that Dortmund will finish in top four but most interesting part will be fight for second place. You are right that there is big difference in strength between Bayern Munich and rest of league. This could give Dortmund chance to finish second if they have strong season. However as you mentioned teams like Leipzig and others are serious competitors. Race for top spots below Bayern is going to be very tough and those teams will definitely be challenge for both Dortmund and Bayern next season.
I think no one has any doubts about the performance of Bayern Munich and about the team's strongest part. Bayern Munich maintain their team in a way that they remain strong in any circumstances. But, it doesn't mean that the other teams are not capable or do not have the potential. There are too many good teams in the German league which are always trying to do their best. Some people say that other teams are not that strong so it will be very easy for Bayern Munich this time to win the Bundesliga. I think Dortmunds struggles are appreciating as they are more willing to perform the best. So, the chances of winning depend on the hard work and the potential of the team. Bayern Munich will win due to their strengths. And the other team also has the capability to fight in a good way.