I was trying to think of which games created certain mechanics that became popular and copied by future games in the industry.
The most famous one that comes to my mind is Assassin’s Creed, with the tower climbing for map information.
I remember them having a sale on Oblivion DLC one time where the rest of the DLC was half-off, but the horse armor was double.
Oblivion was weird on DLC. Knights of the Nine was pretty good, and Shivering Isles was amazing. But they also had bullshit stuff like Horse Armour.