I don't think Dortmund will be a fierce competitor in the Bundesliga title race this season, they have undergone a lot of changes and on the other hand Bundesliga teams like Leverkusen, Leipzig and also Bayern have improved much stronger, it is the opposite of Dortmund so I doubt a little that Dortmund will compete fiercely in the title race especially this season they have a new coach of course every new coach needs to adapt in building a team as well as for Dortmund's new coach this season.
Don't think Dortmund will fall behind that easy. They technically do have a new manager, but Nuri Sahin was always there, he was Dortmund's assistant coach all along and knows the club better than any other manager that could have been brought in. Out of the first two games, he got the team 4 points, which is definitely not bad. The Bundesliga is evolving, a lot of teams are coming out of their shells, and have stepped up their games.
Yes, Leipzig can be really good in the coming days, as they always have a pretty solid squad, but fail to stay consistent. Apart from that, Bayern Munich wants everything back again. Last season was nowhere near their expectations, and they do want to return to winning trophies like they used to, and hence I really think they can keep playing at the highest level from now onwards.
Dortmund should be winning a lot of games in the league from now on, and I don't doubt them too much. Fun part about Bundesliga right now is, a lot of teams tend to be changing their tactics so they can be even better in the playing field, makes everything much more fluid for sure.