Im tired of the Oblivion workout plan that every NPC seems to be hitting. They are just "too" buff. I propose a change for Skyrim. Lets mix and match each NPC to suit their background/story. It's understandable that guards/fighters should be in adequate shape but peasants/bystanders/different shop keepers should have a more varied look. Different weight/height can really make for a more immersive world.
This can affect the player as well. If he/she does not keep themselves in shape, a skill that could be based off athletics, they get a little chubby. Food could also play a role in this as eating butt loads of mutton a day should surely have some side effect...
Even better yet, have each NPCs appearance reflect their athletics skill or whatever skill should be governing said fitness.