Heidenheim is not a weak team, they are strong especially when playing at home and the draw against Dortmund is quite natural because on the other hand Dortmund's strength this season is not that strong and in my opinion it is not even stronger than last season.
If compared to Bayern Munich, Leverkusen, Leipzig, Stuttgart, the Dortmund team is not stronger, so with the fierce competition this season, I quite doubt that Dortmund can finish in the top 4 of the final standings.
heidenheim is a mid-table team that is quite capable of beating dortmund, even though in their last 5 matches they didn't win at all, they almost made 4 draws, in this match the result is most likely a draw.
another interesting match to watch is darmstadt vs leverkusen, this is an important match for leverkusen, the difference in points between leverkusen and bayern is 2, so this is an important match for leverkusen, a draw or lose will have a negative impact on their position on the standings.