I dont think facing Dortmund can be used as a benchmark because after all Dortmund are one of the best clubs at the moment so the draw is actually very good especially with the dominance that Leverkusen showed in this match.
Leverkusen will still be very good if Xabi still shows his slick performance in bringing the boys to a good consistency it's just that for me now even if Leverkusen are like that but this will not make a difference because they will only trouble Bayern for Champions but not to replace the hegemony that Bayern is doing.
Now its all about how they keep the scheme and the players and of course a few more players to keep the rotation going well in a busy schedule could be a very good thing.
Dortmund was the best club agreed,but you should not underestimate the other clubs.Because every club had their own potential in the game.Every club will had the star players who help them to fight at the important matches.The rotation of the players also the important one for the club to chase the goal,because it was the strategy of the Leverkusen to keep their position in the top of the table.Today match for Hoffenheim and VfL was seems to the game of Hoffenheim today.Because they made of 2 goals in the mean time and had their top position in the game.So most probably it was Hoffenheim will win today.
I'm deliberately not discussing other clubs because this is meant to avoid widening the initial discussion where we are discussing Leverkusen who are considered to have declined only because of the draw against Dortmund.
Indeed we know there are several clubs that now need to be paid attention to as you said but actually when talking about the decline of Leverkusen in this case I think this is too exaggerated because however the results that occurred in the match between Leverkusen and Dortmund before cannot be used as a benchmark that Leverkusen has regressed in terms of performance because 1 draw after a series of wins in my opinion is still something very reasonable.