They'll most likely release something closely related to that album, if not a remastered version with some filler, although Nirvana's whole discography has all been remastered recently from the original analog tapes and released in 180 gram vinyl format. You might think it won't make sense to release another one so soon, but one never knows what's going on in the minds of the people who are in charge of all things Nirvana these days.
Surely something will be announced soon. There's an official Nirvana website that has just recently gone online:
At the moment, it doesn't offer any content other than a subscription form for getting newsletters.
One of the benefits of the album's anniversary for us Sonic Youth fans at least, is certainly that "1991: The Year Punk Broke" is gonna be finally released after many years of unfulfilled expectations.
Fantagraphic Books has released a book, "Taking Punk to the Masses: From Nowhere to Nevermind - A Visual History from the Permanent Collection of Experience Music Project", in relation to the exhibit currently taking place at Experience Music Project Museum in Seattle, Washington.
Link to purchase the book:
http://www.fantagraphics.com/index.php?page=shop.product_details&flypage=shop.f lypage&product_id=1993&category_id=16&manufacturer _id=0&option=com_virtuemart&Itemid=62&vmcchk=1&Ite mid=62
Some info about the exhibit: