Me and my bro are big Elder Scrolls fans, I played in order:
Morrowind - 350 hours.
Oblivion - 260.
Skyrim - 150 hours so far.
I would have played Arena and Daggerfall but in all honesty I was too young. Anyway back to the point, my bro and I were discussing what needs to be added to Skyrim. List follows:
Repairing gear - I miss this, although mundane it does make sure you keep your gear in check.
A breath meter - I think its a bit daft that you can't see when you're going to start drowning.
Crafting Arrows - I honestly don't know why this is missing but there you go.
Dragon bone and scale weapons - Need I say more.
A system where by donating money to a Jarl of a city will increase the wealth of the shops in that city, also if you bring rare commodities to a shop that they need it will be worth more than normal.
I bit of a subtraction:
Get rid of Fast Travel - I honestly think that this ruins the immersion of the game sometimes and I end up being lazy. On my second toon I have forbid myself from FTing anywhere. The carriages are more than enough to get from city to city and then just walk or ride from there.
That's all I've got so far but please let me know what you guys think below

