Elder Scrolls IV: Oblivion
1) Revamp the main storyline. Mainly, make it less of 'the world is in your hands!' type of story. Make it more low key, to fit with the rest of the changes.
2) Revamp the dungeons. Whoever thought having 500 dungeons with only 4 or 5 different designs was an idiot. Make each dungeon unique and interesting, or at least make them not look exactly like every other one.
3) Revamp the world map. The game takes place in one province. That's cool, so did the last one. Except this province is way, way smaller. Make the game stretch all the way the country, make it include the Summerset Isles. Make it awesome. If needed, make the game have multiple discs to store all of it. (Also, as an aside, let the character take ships places. I don't need to control the boat, but I want to be able to hang out on it while someone else does)
4) Revamp how quests work. First of all, pump up the available factions you can join by a ton. Make a random quest generator for each guild in addition to the story missions.
5) Revamp the enemies. The enemies here are good, but we need more especially considering how bigger the scale would be. Add in more undead, more goblins, more types of people to fight, more animals, more everything! Variety is the spice of life!
6) Revamp the equipment. Not much to say here, just make a crapload more weapons and armor and whatnot. Also, for enchanted equipment? Make it actually look sort of different, at least. Not this 'different colour when you unsheathe it' nonsense you've got going.
7) Revamp the soundtrack. Seriously. It has a good sound, but it has so few songs! And there isn't any unique songs for unique places! This is bad. Fix it.