Expansion Packs are for MMOs Only, and only unlockable after sufficient research.
Expansions are supposed to be developed when an MMO's maintenance costs begin to outweigh its income; if your MMO game is beginning to be unprofitable, and you don't want to go through the effort of creating a new one, an Expansion Pack can be useful.
Expansion Packs can be created if you have researched MMO Support and Expansion Packs. They increase the profit of the MMO without decreasing the expenditure/creating a new sales graph. Expansion packs are individually made, reviewed, and marketed but are still part of the original MMO. You cannot be requested to make an expansion pack by a publisher. *Note* Expansion packs do not act like sequels, you will not get bad reviews and low sales if you make an expansion pack without placing a different genre/topic game in between.