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.