Long War (pictured above), the immensely popular mod for XCOM: Enemy Unknown and XCOM: Enemy Within, will get a sequel in XCOM 2 called Long War 2.
The mod makers were formally brought aboard XCOM 2 before its February 2016 launch and developed three mods for it as Long War Studios. The third and last will be Long War 2; it also will be free.
Additionally, Long War Studios has rechristened itself Pavonis Interactive. It's also working on a totally separate strategy game called Terra Invicta, which is under development in anticipation of a Kickstarter campaign.
Otherwise, there aren't many details to share about Long War 2, other than Pavonis and XCOM 2 developer Firaxis promise to divulge more in the weeks ahead.
Long War was very well received by the community, critics, and XCOM developers themselves and is seen as the reason XCOM 2 would offer mod support and a toolkit from day one.
The Pavonis developers have already put out two free DLC packs under the arrangement, the Alien Pack and the Perk and Laser Pack. The Perk and Laser Pack was available for XCOM 2 on day one. They also delivered the Toolbox mod back in April.
Long War is a conversion mod that added a slew of new options, features and content to XCOM: Enemy Unknown and Enemy Within. Among other things, it expanded an XCOM fireteam size to 12; added several soldier and MEC classes, several perks; mission fatigue and other variables; improvements to the UFO intercept sequences, and tons more. It also, per its name, greatly lengthened the game's campaign.
Polygon went in-depth with Long War in 2014, about a year after its initial release. The final version of Long War was released at the end of 2015. It's available through NexusMods.