Long War was one of the first XCOM reboot's most popular and beloved mods, so much so that XCOM 2 ended up liberally cribbing notes from its playbook. Now there's a Long War for XCOM 2, and it's a doozy.
The goal of the mod is to make players feel like they're waging a real fictional guerrilla war against aliens. Long War 2 plays up the infiltrator angle. Your max squad size is 10 (versus vanilla XCOM 2's six), but smaller squads infiltrate areas more quickly, and an effective infiltration means weaker enemies once you're on the ground. It's a cool dynamic that means big squads of scrubs and small squads of commando killing machines are both viable, depending on the situation.
That's hardly all the mod adds. In some ways, it really is a whole new game built inside XCOM 2. Here's the feature list:
1) A much longer campaign, running for 100 to 120 missions on average
2) Infiltration mechanics that require you to send out multiple squads at once
3) Manage resistance Havens and have resistance members scrounge for supplies, gather intelligence or recruit more people to your cause
4) Nine soldier classes: Sharpshooter, Shinobi, Ranger, Assault, Gunner, Grenadier, Specialist, Technical and Psionic (plus Sparks for DLC owners), each with unique secondary weapons
5) Two new weapon tiers, lasers and coilguns
6) Dozens of new enemies and smarter battlefield AI
7) ADVENT strategic AI that tries to counter your moves
There are also new mission types like prison breaks, and many other options besides.
It's a bigger and slower take on XCOM 2, but it's also arguably a smarter, more varied one. Basically, if you've been looking for a reason to fry more aliens (and also your brain) after you took a break to fry other things, I can't think of a better single reason than this mod. Now get out there and give those hollow-eyed motherfuckers hell. Long hell.