A photo allegedly taken during an internal meeting at Bungie appears to list the company's plans for the next two years of Destiny. Neither Bungie nor Activision have verified the image's legitimacy, but it appears to be the real deal. More importantly, the plans it details just make sense.
The image quality is poor, so it's hard to make out all of the details, but here it is followed by a quick summary of Bungie's master plan for Destiny.
On March 10, 2015, the second expansion pack will release. It's called House of Wolves. We don't know what its story is about, but it's expected to be comparable to The Dark Below, with new missions, strikes, Crucible maps, a Raid, and loads of new gear. It will probably center on the Fallen.
Sometime in September of 2015 (coinciding with Destiny's one-year anniversary), Bungie is planning to drop a massive expansion pack so big, it doesn't sound anything like your typical DLC. It's titled Plague of Darkness, and the image reveals that PoD will be filled with enough content to make it almost as big as the original game: a whopping 12 story missions, new strikes, one new Raid, a new Patrol area called "Hive Ship," as well as some big additions to gameplay like three new sub-classes and two new weapon types.
A collected "Game of the Year" edition that includes Destiny and all three expansion packs is expected to arrive in stores at this same time. It's codenamed "Comet."
Around March of 2016, a fourth expansion pack will be released. Its title isn't quite legible on the image, but you can see the word "Vex" in it. Expect the usual new content, likely this time focused on the titular Vex.
Summer(ish) 2016 will bring the final expansion pack for the original Destiny. The image calls it Forge of Gods, and by process of elimination, it will should feature the Cabal as its main villains.
Sometime in late 2016 or early 2017, Activision will release the full sequel, Destiny 2. (Bungie is contractually obligated to turn in a sequel in this timeframe.)
The leaked image originated from a post on the official Bungie forums.
