I think a great addition to DDO would include some seasonal either real world or Ebberon related quests that are only available during that season would be unique. It would make the quests valuable to have for the favor and add more feel to the quest. I like the idea that you cant go and run the quest until a certain day or month because, well lets face it, everything isn't always available.

Maybe you could include some new quests that involve protecting merchants while they move there goods, similar to the Desert Caravan quest, but have them only available on specific days or weeks or months. These do not have to be seasonal as merchants are frequently replenishing stocks. These quests could also be scaled as the Devils Assault quest is scaled and an epic version to satisfy the level 20's. This way all players of all levels could benefit for the addition of new quests.