I would leave it mostly like it is except:
Remove XP penalty from classes
Make Archery for Rangers viable dps
Give Rogues the ability to make traps
Give Rogues actual traps to detect and disarm in the game world
Have more doors require Lockpick
Give warriors a Block skill that requires a shield
Give warriors more abilities / take kick/bash off the same timer + buff bash damage with a shield
Buff Wizard nuke damage and mana ratio (and maybe give them spell crits, or nerf mage nuke dmg/ratio)
Not a whole lot really. The farther the game got from its DnD roots the worse it became imo.
edit: also, with modern AI, I wouldn't have dragons/gods acting like stupid mobs who just stand there and get beat on. Spoony says it better than I could:
http://spoonyexperiment.com/2012/09/...circle-strafe/