In a surprising development, it seems that the “third” entry in the fifth generation of Pokemon games will actually be two separate titles, releasing simultaneously. As mentioned below, they will be called Pokemon Black 2 and Pokemon White 2 and they will release in Japan this June. No news on a North American release, but the U.S. site says they will be “coming soon.”
The U.S. site also calls the games “two incredible new adventures” - which, paired with the Black 2 and White 2 branding, may suggest they are actually direct sequels of sorts to Black and White. To be clear, this has not been confirmed and is merely speculation. No official word has been given on whether it will feature the same story and areas as Black and White (as is traditional for these third entries) or whether it will somehow continue the adventure. Keep it here on IGN for all the latest.
The official Pokemon japanese website has been updated to include news of this new game. It mentions that “mysterious legendary Pokemon” will make an appearance. The images of these Pokemon (below) look like new forms of Black and White’s legendaries Reshiram and Zekrom, perhaps somehow fused with Kyurem. Of course, only time will tell who they are for sure.