I cannot really vouch for Frankfurt at the moment due to their inconsistent run so far, though it seems like they are the team that poses some threat to Frankfurt in the league right now and they haven't been their best this season compared to previous seasons when they were in good form, but maybe they might just put their house together and start getting the right result but I still do not see them as a potential threat to the leverkusen side, at least not at the moment.
Leverkusen could be an obstacle for frankfurt in terms of competing for a better position in the Bundesliga standings this season, although they are currently above leverkusen, but honestly it will be a very difficult job for them to maintain their position from leverkusen at this time, because to doing so they must immediately improve their consistency which so far is still unstable, they have lost two and drawn three of their 13 Bundesliga games this season, so although they seem to be a team that could pose a threat to Munich in the league, but I don't think they are ready to challenge for the title with Munich although if they has show their best form this season.
The difference here is that Bayer Leverkusen are capable of handling most of the big teams in the bundesliga whereas the same cannot be said about Frankfurt and at such it would be very easy for the Leverkusen side to suddenly overtake Frankfurt soon enough as the season progresses. Frankfurt's inconsistency will be a determining factor and an advantage for Leverkusen who might take advantage of Frankfurt's mistakes this season to go above them because as it is right now in the bundesliga, the only team that can compete favourites against Bayern Munich is the Leverkusen side.