Make the ability to enchant stuff like the morrow: I like how your weapon could be as powerful as long as you have the gold for it.
Bring back the gods: I like how the could go and fight Vivec and actually have a challange ( even though you werent supposed to lol)
Bring back the houses: I loved being apart of the televina house and having a cool tower.
bring back the book of rare weapons: I liked going and being able to find a weapon, ring or armor, and know if it was common or not.
Have more SIDEQUESTS! I was told it would take 3-5 months to beat obivion, it only took me a month to beat it 100%
What to keep:
Keep the arrow from obilivion: I do admit it was hard to do the quest in morrowind with no arrow, and I think thats what people hated about it.
Keep the fighting system: Both are really the same but good.
Keep the dark brotherhood quest: they were the most fun
Conclusion:
That is all i can think about right now if i have more then i will comment later.