River is wearing the eyepatch lady's patch.

Yeahbuhwhat?

It's entirely possible that this is a tremendous red herring, given that the trailer shows it so plainly. If it's not, then I have no idea what's going on.

We can theorize that River can (or used to be able to) regenerate, but we also know that this version of River dies in the library in Series Four, making it seemingly impossible for this version of River to age into the eyepatch lady. The eyepatch lady hates the Doctor (or pretends to), and River has always been a big Doctor supporter.

There is an admitted physical resemblance between River and the eyepatch lady. And there would be a nice symmetry to having baby River, adult River, and old lady River all playing a part in this storyline. However, if River is eyepatch lady, then she wouldn't be able to touch the baby version of herself. It was established in Series One via Rose that touching yourself as a baby is a Bad Idea. Has eyepatch lady touched the baby? I can't actually remember.

The only explanation that I can think of goes something like this:

Because River is only part Time Lord, she has an odd version of regeneration. Instead of regenerating into a whole new person, she regenerates into a younger version of herself, healing herself of whatever ills or fatal wounds she's suffered. Our version of River is thus not the same version of River that we see in Silence in the Library. Our current version of River will grow older and become the eyepatch lady, for some reason pitting herself against the Doctor -- perhaps because something in her changes, perhaps she is influenced by something, perhaps she does it because an older version of the Doctor tells her it needs to be done, perhaps she figures out on her own that it needs to be done, perhaps something else. Who knows.

She kidnaps Amy (her own mother), keeping her isolated until she gives birth to baby River, and then sets up all this stuff with the Doctor. She does something bad. She kills the greatest man she has ever known (either the Doctor or, perhaps more likely, Rory). She regenerates into a younger version of herself and is incarcerated in that prison for the crimes she committed as the eyepatch lady.

If she is able to regenerate into a younger version of herself, perhaps that could even provide a canonical explanation for Alex Kingston (River's actor) being younger in Silence in the Library than she is now.

EDIT - It is also possible that eyepatch lady is an older version of a previous incarnation of River, and that once defeated she regenerates into the Alex Kingston River that we know. But that doesn't necessarily explain Alex Kingston River wearing the eyepatch.