Looks like Merc is pursuing Bottas, but Williams wants a good chunk of money and an experienced driver to pair with their new rookie.
Talk is about un-retiring Massa (apparently he only retired because it looked like he was out at Williams and couldn't line up another quality drive) but I think Button is more likely to be in a Williams next year. I remember other rumors about a return to Williams for a swan song...
Then again who knows how tight Mclaren wants to hold onto him, guess it depends how confident they are in their rookie. There's a chance he gets called up to sit in Macca's seat, I still remember the year Ferrari had to replace Massa, not pretty for awhile there.
http://www.bbc.com/sport/formula1/38335169