I see a lot of different possibilities.
1. Madara could be the last villian, just because he is behind AK, and he could possibly be the reason behind the Kyuubi (or is the reason). I could see him having another confrontation with the Kyuubi somehow.
2. Zetsu is mysterious and all so we don't know exactly what to expect from him.
3. Itachi is so mysterious to, and it seems that he has some kind of plan up his sleve that no one knows about.
4. And lastly,Danzou wants to take over Konoha or something because he is a sore loser to Sarutobi. Having Kabuchimaru on his side would help a lot in doing this, but Kabuchimaru is like that unknown thatwe don't know what he will do next.
I think that it will mostly be 1 for the first option. After that, 4 would be the second option. I don't see 2 and 3 being with the other two.