Madara is operating solely based on his own motives. I think Kabuto confirmed this when he first summoned him. Regardless, though, there are still a number of individuals that not only likely don't like the fact that Madara is "alive", but also have the ability to kill/beat him.
1. Tobi was noticeably upset when Kabuto first summoned Madara. I don't know if Tobi could actually go up against Madara, since we haven't really seen Tobi pushed to his limits, but I think it would be a good match.
2. I think that Sasuke was happier when he thought that he was the last Uchiha. Plus, I don't get the impression that he really likes Tobi that much anyway. That being said, Sasuke is not really all that predictable in terms of who he likes and doesn't like, and I don't think Sasuke and Madara are going to get along. Madara even acknowledged Sasuke when he was first summoned and noticed an extremely powerful chakra. Since he and Kabuto were talking about Sasuke at the time, I assume that he was noticing Sasuke's chakra, but then again Madara could have been referring to Tobi, Itachi, or even Naruto at the time. Regardless, Sasuke is going to surpass Madara and will eventually be able to take him down.
3. Naruto is by far the most powerful character in the series at the moment. He could probably take on Madara with some backup. Namely, Shikamaru or Kakashi to develop his strategies. Other than that, he has the raw power to probably go up against the likes of Madara by himself.