UPDATE: Added an 'official' (presumably) source. Also learned of extra new details (in italics). Information is originally provided by Kotaku.
The single-player campaign is returning back to the US, including major cities like New York and Manhattan. Paris and London are two other confirmed locations, but concluding from the list, there should be a large list of capital or major cities. The final mission is speculated to be located in Dubai, but it's unclear whether or not this is true or not. Multiplayer maps are usually based on the single-player campaign, so be wary.
Also, it's believed we're going to have two new protagonists. Their nicknames, 'Frost' and 'Sandman' are expected to debut with their new storyline, but the major protagonists, Price and Soap Mactavish, as well as Nikolai, are making a return. Modern Warfare 3 is expected to wrap up all the loose ends, and if that's true, it means that Makarov is going to return with a major part in the story.
There is also speculation that this, as possibly the last link a trilogy, may be Infinity Ward's last Call of Duty game.
Anything you'd like to say to this?