How's this for a crazy theory. What if Madara is actually "creating" a new type of "tailed beast" in Sasuke, probably conveniently called "Zero-Tails" (Japanese seem to love that word), and is actually needed in order to do that 10 tail thingy he mentioned earlier?

In a way, all the Karin talk about his evil chakra, and how it is just like Naruto's Kyuubi would make sense. It would also make sense why Madara is driving Sasuke into all this mayhem. It would also explain the huge chakra reserves that Sasuke seems to have gotten out of nowhere. It would also kinda explain that huge dark figure that appeared once behind Sasuke in one of those chapters.

Dunno, just thought this out after reading some of the comments in this thread. Don't bash me for it.