Bundesliga can be categorized as a competitive league. The weak teams you mentioned do not mean that they cannot provide a surprise, including Heidenheim, they could show it later at Signal Iduna Park, and I do not agree with the definite guarantee that you said. If it is true that Dortmund will definitely win, does that mean Heidenheim cannot win? How do bookies provide odds if victory is certain in Dortmund's hands, and I as a bettor would be happy if it was guaranteed. 
In my opinion, both teams have a chance as long as they can maximize the existing opportunities by focusing and working hard in the match.
Heidenheim have good start in this season after two consecutives winning against St Pauli and Ausgburg but not sure until how many games weeks later keep consistent with their performance on the winning trend. In my opinion, both teams have a chance as long as they can maximize the existing opportunities by focusing and working hard in the match.
Upcoming match will face difficult team Dortmund and waiting will success continue the winning trend or Bundesliga dominate only by top teams such as Bayern Munich, Dortmund and Bayern Leverkusen.
Bayern Munich keep get consistent performance after two consecutive winning under Vincent Kompany era, bit unbelievable since appointing as manager many people expected with Kompany get failure bring back Bayern Munich to the Bundesliga tittle race. Until games week 2, only three teams success winning two matches from Heidenheim, Bayern Munich and RB Leipzig.