You should be able to cast fire spells to keep you warm and frost spells to chill enemies

I personally like the idea of adding more realism to open-world games like Skyrim. I liked the hardcoe mode in Fallout: New Vegas, so I would like to see some more realism added to Skyrim. Most of these points are already mentioned, but here's what I was thinking:

• Body temperature stat
  
• All apparel adds a rated buff / protection to body temperature
  
• Outside temperature based on time of day and regions / elevation
  
• Staying outside for X time decreases body temperature based on outside temperature
  
• Swimming (as someone mentioned) also decreases body temperature
  
• Since weather (snow and rain) is wet, it increases the rate of chill
  
• Be inside a building to recover your body temperature
  
• Be close to a fire to speed up recover of your body temperature
  
• Ability to use firewood in inventory with fire spell to create campfire outside anywhere - its recover rate based on where are you

And from Fallout: New Vegas' hardcoe, I would definitely like to see at least sleep and food (and I wouldn't mind water) added to Skyrim as well. Skyrim is a harsh, cold world. A little survival realism would be greatly enjoyed by some players.

I think that you can add lots of realism without making things overly complicated. Skyrim has already been simplified from previous Bethesda games in terms of what players expect from an RPG. The general temperature system I've described would be fun to have without being too complicated.