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.
In my own opinion, Bayern Munich have been the big deal in that league for decades now apart from season before last that Bayern Leverkusen won it so definitely Bayern Munich will win it while Dortmund will come second judging from their performance lately that's why I'm considering them as the runner up I'd say Leipzig but those guys has really let go of some of their key players so I'm not really sure of them that why I'm putting Dortmund there, then third place is Leipzig but they won't find it so easy to be there they will definitely fight for it, fourth place should be Bayern Leverkusen if they did well, because with Erik Ten Hang as the head coach I'm not seeing any future with him, so if they try they might secure the fourth place position, this is my view.