Last year the circumstances were also very special in the EL. This year they are in a neat 4th place in the league, and the differences with the teams above and below are minimal. They could therefore still be 2nd in this competition. It is the club with the most self-confidence after Bayern Munich. Because how much confidence should you have if you have wintered in the Cl and that you are also the current title holder of the EL?
For the second position in the Bundesliga it is still very possible for Frankfurt in terms of getting it. But for the Champions League, I see Frankfurt getting a very tough opponent this season, namely Napoli who have never lost this season in Serie A. So Frankfurt has a very tough task this season, especially in the Champions League because Frankfurt needs to really focused and consistent in order to beat Napoli in the Champions League this season.
I think top 4 is a likely finish for sure, but second position I still nominate Dortmund there. They seem like they are working to fix some things and get some players in January transfer window, and I am guessing that they will play a lot better and could be second. UCL on the other hand I agree that Napaoli is a much better team this year, and they will probably win.
It's not going to be so easy as to just make it work for them, there is a chance we may not end up with a good game for Napoli as well, just because they haven't lost doesn't mean they won't, if Frankfurt plays right, they do have a chance to win this game and shock the entire world as well.