After a careful analysis of the Series 6 trailer I have found some of the Monsters and Villains for Series 6. Firstly two groups of historic figures will be appearing. Musketeers and Nazis.
Also we see Ood returning for the fourth time, and the first for the Eleventh Doctor. They were first seen when the beast took over a space base. Coming back to that, we saw in one of the clips that Amy gets weird markings all around her face, that look like the ones Toby Zed got when the Ood first appeared.
Only time will tell what the story line is.