I'm not sure if even that Nemesis system is really something completely new. You could probably get a similar emergent behaviour from older systems like goal-oriented action planning, and I wouldn't be surprised if games like Dwarf Fortress already had something similar.
That's probably true, but the specific implementation in the Mordor games is pretty unique. It's not just emergent gameplay, there's elements of deliberate action in there. The game selects certain enemies to become "resurrected" to come back stronger and become your longterm rivals.
The enemies are also generated with perks and flaws based on the available game mechanics. These orcs return to fight you again, they often have upgrades themselves to resist or be immune to the ways you killed them previously.
Taken individually none of these mechanics are novel I don't think. But this particular implementation is pretty unique. I'd love to see similar things in other games. Like Mech pilot rivalries in a Mecha game or something similar. But games aren't copying it, despite how well received it was. I can only think it's due to some kind of patent licensing issue.
