For those of you not in the know or who haven't played Middle-Earth: Shadow of Mordor, the Nemesis System is one that was specifically developed for the epic fantasy RPG whereby the orcs would remember you for what you've done to them, even much later in the game, and retain the scars of your previous encounter. It created a complex relationship structure in the game that made it much more realistic (as much as a fantasy game can be) and drove in into higher consequence platforms.
XCOM 2 designer, Jake Solomon, informed Kotaku during an E3 2017 interview that the expansion, War of the Chosen, would be using the same type of system. The development team were inspired by Shadow of Mordor when creating the expansion, and so brought many elements of the Nemesis System into it.
We used [Shadow of Mordor] as a jumping off point. You’re playing that game, you’re developing these relationships, and it’s just so gratifying to have a game acknowledge what you’re doing as a player. Even to the point where, another element that inspired us from Shadow of Mordor [was the way orcs gain characteristics when you encounter them multiple times]. In XCOM, you fight these Chosen aliens multiple times, and as they get stronger, they gain procedural strengths and weaknesses.
So they won't be featuring any orcs, but the enemy aliens will behave in the same manner. Solomon added that they want to add as much personality to the game as possible, not just with the enemy aliens but with your soldiers too. They can become quirky or exhausted, to the point where they "freak out" when faced with an enemy or specialised weapon. Aliens and soldiers can pick up valuable traits that will assist them in future battles too.
If they’re on a mission together, the bondmate is gonna go berserk. They don’t like seeing their friend die at all. It’s not good.
This is going to make for some intense gaming, I believe. To have such open relationships in-game, where soldiers rely on each other for strength, will be immense. It looks like we are well on our way to see an expansion that will completely overshadow (pun intended) the original game. I for one am glad to see more games take up the Nemesis System, and hope this will be the leading example to other developers out there.