Hmm... Lets see, I would like to see:
Chase Mechanics: There isn't a real feel or sense of tension/action with the old rules. I would really like to see chase mechanics done in a way that each chase is different and unique.
Better Mounted/Vehicle Mechanics: Parts of this should work into Chase Mechanics as well as well as show off the benefits/hazards of being mounted/in a vehicle, ie: being hard to stop on a dime.
Flaws/Psychological Effects: Not every hero is 6'5" and has Iron Will. Many heroes have flaws both physically and mentally, ie: Raistilin. I would love too see as well rules garnering Morality/Psychological effects, similar to the Storytelling system.
Better Battlefield/Mental Effects: I would love to see better mechanics for things like being stunned, frightened, stumbling, wounded, etc.
Fighting Styles: Actual Fighting Styles, perhaps as minor-paths one could take. Different from ordinary feats, so possibly taken on the side. They would be something you invest into, so... Perhaps it comes straight out of your XP (this could be roleplayed as after training you take out of your XP (you gained from training) to learn the next level of that Fighting Style). The amount of XP needed would go up as they become more powerful.
Stunt Rules: Skill Tricks were a nice-try but I would like to see stunts more solidly integrated into the rules. Perhaps it could be worked be certain Stunts have a DC to do them, this DC could be bonused by the proper attribute + skill attributed to the Stunt.
All I can think of right now.