This post is about the new factions that this mod will introduce. It contains a few spoilers, so be warned before reading it.
MJ-12
This faction is very similar to robin's Men In Black mod, being a secret group with its own agenda involving Earth and the aliens. But my main inspiration for this faction comes from its role in the Unknown Menace, the 692 pages fanfic I wrote about the original game a long time ago, and MJ-12 was also a very important piece of UFO folklore in the 1990s.
Without spoiling too much, MJ-12 is a highly professional and secret organization, whose research can be ahead of XCom's. Another way I see them is as XCom's evil twin. As the game progresses, you'll see them using better weapons and engaging on more complex activities. They use a set of weaponry where the weapons look very similar graphically, since they were all developed together (recovered items can be used by XCom straight away, so that the player can also have those really nice looking guns).
They fly around in their own Skyranger transports, which can be easily be destroyed by your interceptors when detected, but you may want to consider letting them land. Besides the weapons, captured and interrogated MJ-12 personnel can unlock some critical alien research for XCom, since their own scientists have already been working on advanced weaponry for a while. Their missions will also take place in some new settings, such as their Skyranger also being present on the map, plus new maps I will create later. And to find out more about the organization, you can always capture their higher ranking officers.
Cultists
The clear inspiration for those is the Cult of Sirius from XCom: Apocalypse, being a human fifth column that sees the aliens as saviors. According to their initial description, "The aliens have had a profound effect on human society, with new and existing groups appearing as a result of the invasion. While most of these groups are benign or harmless in nature, others are clearly looking to change the old balance of power on Earth and will take any means necessary to achieve their goals, including becoming allies with the aliens. The cultist movement propaganda poses their actions as a crusade against the corruption of Earth and even gives religious overtones to the alien's appearance. While recognizing its threat, Earth's governments have chosen not to get involved in an effort not to lend credibility to their claims, and have assigned XCom to privately deal with the cultists".
During missions Cultists will initially be more like a civilian mob, being armed with everything they can lay their hands on (Pistols, AK-47s, Hunting Rifles, etc.). But as the game progresses, their alien 'friends' will take notice and start equipping them with advanced tech, and even using them during their own missions. Cultists will also fly around in Transport Planes, have their specific maps, and capturing some of its members can unlock key research topics.
Other Aliens
Finally, it will be possible to encounter new alien races. Those aliens are not allied with the main aliens but they have taken notice of Earth because of the invasion and have also decided to join the party. They will fly around in their own UFOs and it is also possible to unlock key research topics through them. I won't reveal more but these aliens will basically pop up unexpectedly and they can be quite a challenge.
