Welp, I'm going to try and list some elements from
every Elder Scrolls installment (sans Battlespire/Redguard) that should be re-implemented in some form:
Arena: Local temples. I really liked being able to visit these to be cured of diseases and donate to receive blessings. Would be pretty fitting in Skyrim, as you could have a number of them from Imperial/Nordic pantheon and pagan denominations.
Daggerfall: Expansive cities are an obvious must for TES V. At least some of the older ones such as Winterhold ought to be fairly populous. I'd like to see the banking system return as well, and possibly some degree of randomly generated quests/assignments so that you always have something to keep yourself busy.
Morrowind: Immersive, lovingly hand-crafted world with minimal level scaling. Political intrigue and a strong level of cultural influence on the world should be prominent as well. Oh and one more thing - lockpicks of different qualities, with multiple uses!
Oblivion: Radiant AI (obviously), awesome physics-based dungeon traps, and lots of scenery porm. But without the bloom.