Dortmund will only be in the top four next season, but there's little belief that Dortmund might finish second next season if other teams don't perform significantly better. Since there aren't many teams in the Bundesliga with the same strength as Bayern Munich, Dortmund could have a chance to finish second next season. However, teams like Leipzig and several others could pose serious obstacles for Dortmund and Bayern Munich next season.
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.