IMO, Dortmund mostly becomes the biggest rival of Bayern Munich, they often finished as a runner up. Leverkusen only became stronger in the last 2 seasons, they rarely finished in the top 3 in the past. Nowadays, I think we have 5 strong teams to be the potential competitors of Bayern Munich in the next season (Leverkusen, Dortmund, Leipzig, Frankfurt, Freiburg). These teams look improving their squad gradually in the last few seasons. I'm sure they have a chance to be stronger teams in the future.
The strongest teams in the Bundesliga were Munich and Dortmund. Dortmund managed to find some talented young players. However, they did not want to dominate the Bundesliga or win the title. Their main target was to make money by selling young talented players. That is why we are not getting a better performance from Dortmund this season.
In the last two seasons, we have seen that it is gradually becoming difficult for Bayern Munich to win the Bundesliga title. Teams like Leverkusen, Leipzig and Dortmund are challenging Bayern Munich. Hopefully, the Bundesliga will become more competitive next season.