http://www.gamespot.com/articles/destiny-2-launching-in-2017-large-new-expansion-co/1100-6434724/

Destiny will receive an expansion in 2016, followed by a full sequel in 2017, Activision announced today.

Few details are available presently, but Activision describes this year's expansion as a "large new" one. This suggests it could be on par with The Taken King, rather than the smaller Dark Below or House of Wolves DLC releases. That's merely speculation at this point, however.

There's even less to go on regarding the sequel. Activision calls it a "full game sequel" that's due out in 2017.