I will not put any hope in Borussia Dortmund challenging for the Bundesliga title next season because Borussia Dortmund is not always consistent in maintaining the performance that will make them challenge for the league title. If not for Bayern Leverkusen in the Bundesliga this season, Bayern Munich would have ended up winning the Bundesliga title. Until Borussia Dortmund proves otherwise, next season's Bundesliga title race is between Bayern Munich and Bayern Leverkusen. When the transfer window opens, Bayern Munich will not only sign players but will also be careful to sign players who will be productive and help the club take its place in the Bundesliga League.
If not Leverkusen, maybe Stuttgart will be a contender for the Bundesliga title this season because they are performing well and they can even finish in 2nd place, although perhaps towards the end of the season Bayern Munich's performance will no longer be as optimal and it might be a different story if there is no Leverkusen name at the top of the standings.
Usually every season Borussia Dortmund is the main rival for Bayern Munich to challenge for the Bundesliga title, to be precise in the previous season they were one step closer to becoming champions if they could win their last match in the Bundesliga, but unfortunately they couldn't win, so Bayern Munich became champions again. I don't expect much from them, but I hope they can apply very tight pressure.