Leverkusen didn't lose any game after 7 rounds and they have only one draw while Bayern got 2 draws and no defeat making Leverkusen in the first rank with 2 points difference from Bayern Munich.
I find Stuttgart interesting too although I doubt they could keep their position at the opposite of Leverkusen.
There is nothing impossible for Stuttgart to become champions in the Bundesliga this year, if I see that Stuttgart has been quite consistent in playing the last 7 matches. Even though they have experienced 1 defeat, apart from that, Stuttgart has a player who is quite consistent in his game, namely Serhou Guirassy as a striker, and has been the top scorer in the last 7 matches. I think Stuttgart will enliven the competition in the Bundesliga in fighting for the title this year, although it is still too early to draw conclusions now.
No, it's too big for Stuttgart even though they are one of the clubs that gave a very big surprise at the beginning of this season but to become a champion in the Bundesliga is not as simple as it seems. Some clubs like Dortmund and Leipzig who are always consistently in the top 4 are not able to realise that let alone Stuttgart.
I think they also know that they will not be able to do that so instead of trying to aim for the championship I think Stuttgart will just do the best they can with the consistency they currently have because this will be very good for their growth and at least with the current conditions they are not bothered to get out of the relegation zone like last season.