Personally, I would be delighted if the DLC introduced seasonal changes. I loved that in the GameJam video and it would build on one of Skyrim's core strengths - the beauty of the game's landscape.
I really like the idea about adding seasons and holidays to the game. If the cities and villages react accordingly with decorations and some small festivities, and a lot of new lines for the npcs, this could become quite interesting. It would bring some diversity, and you'd have to listen less to the same old lines the npcs keep on saying. Plus, holidays and quests go hand in hand!
Well, let's first see what Dawnguard brings, shall we? I'd better go back to my usual 'no expectations' state, so I can't be disappointed. Although, the Snow Prince hypothesis is quite nice ...