Now this might have already been mentioned but here is the idea. Wouldn't it be nice if weather could actually affect how you experience the game? For example, you are in Cyrodiil going for a siege and fog rolls in, it could affect the view distance of characters making it possible for two large group to pass each other without notice (potentially). Or during a storm your travel speed could be slightly reduced due to muddy terrain. Also, what about underground dungeon, why is it so bright? It could be dark enough that even with the brightness setting you would require a skill like Inner Light or there could be an automatic animation of your character pulling out a torch thus repelling the darkness.
What's your opinion? Is it too unrealistic to think it would be possible? What other environmental effects could be interesting?