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 will not be surprised if eventually Bayern Munich looses out on this seasons Bundesliga title but with leipzig, and Dortmund I do not have a particular, they both frequent the top four positions on the table often, it will be almost like a norm if it happens that both of them eventually finishes top four or even either of them but with the title Leverkusen even looks more viable than this other two for the title.
I would be very surprised if Bayern Munich does not retain the league title this season because all the teams that are currently Bayern Munich's opponents in the German league have not been performing very well recently, so it will be relatively easy for Bayern Munich to retain the German league title this season. Bayern Munich's performance can be doubted in some aspects because this season Bayern Munich's important players have left for other clubs, on the contrary, Bayern Munich has not recruited any good quality players in their place. If multiple injuries occur in the current Bayern Munich team after the season starts, it may have a bad effect on their good performance, which will show that they are not able to perform as well as they are supposed to.
Hopefully, such an accident will not happen to Bayern Munich and they will finish the German league with 100% success next season.