Skyrim is probably the game I enjoyed working on the most at Bethesda. Not only because it was the first Elder Scrolls game I worked on from beginning to end, but because it was the one where I had the most opportunity to add my own touch.
Along with significantly improving the Pathfinding system I had begun work on for Fallout 3 (including a complete overhaul of how it interacted with the animation system, or how it handled quadrupeds), I was able to add a completely new aspect to the game itself, namely the critter system.
Butterflies, Dragonflies or fish weren't supposed to be in the game, or if they were going to be, they would simply be animated meshes, because the actor system of the game couldn't support that many new intractable entities in the world.
I created a small actor system that relied almost entirely on scripting for lightweight processing. This way, butterflies to would have their own (extremely simple) AI and interact with the world, landing on objects, running away from the player, etc... all under the control of designers and artists. By removing the hurdle of needing programmer time to add new behaviors, the critter system spread like wildfire and pretty quickly every world artist in the company was adding small animals to the world.