Most likely this is going to address (SPOILER)the twelve regenerations limit, how he survived Trenzalore and what happened there, what caused the destruction of the TARDIS in The Pandorica Opens and why it was destroyed, what "Silence Will Fall" means and why the Doctor doesn't know what it means, the Doctor's Fear from The God Complex, and likely things I don't know/remember.

Off course, Moffat has also said there will be a cliffhanger so

but then again Moffat lies, so who knows really