Comet

Comets are (or were) four planned expansion packs for the Destiny franchise, one for each game, and released in the year between the games. Due to this, the first expected Comet was to be released in 2015. The others were expected to be released in 2017, 2019 and 2021. It has since been stated that the first Destiny game will receive two expansion packs. It is unclear whether this is a divergence from the original Comet concept, or a different type of expansion.

Trivia
A supposedly leaked slate from a Bungie conference labels an expansion titled "Comet: Plague of Darkness," to be released in 2015. This expansion will supposedly feature new subclasses, weapon types, Crucible maps, story missions, and locations.