So Leverkusen's 35-match league unbeaten run ended by Leipzig.

For the Bundesliga, yes. Finally, the unbetaen record by Leverkusen was broken. Leipzig, fortunately this is also one of the top Bundesliga clubs, so it's not so embarrassing. Now, their focus is to defend the Bundesliga title.
But, can they?
For now, Leverksuen is in 8th position. It's still very early. However, their main competitor, Bayern Munich, is actually getting better and more consistent in their game.
On the other hand, Heidenheim is currently quite popular, topping the standings, never conceding a goal, and scoring the most goals.

Next week, they will face a top club, Dortmund, can they beat Dortmund? If so, then Heidenheim is truly the next promoting club that attracts attention this season.
It will be quite a tough match.
H2h:

0 win, 2 draws, 0 loss
Source: Heidenheim vs Dortmund