Like was done by Adam Miller, reduce creature XP, provide more XP through quests giving better control over leveling so later encounters can be planned for properly.
Redesign the henchmen, they were made for before HOTU when you could only have 1, now you can have 2. Give them some interaction with each other and the PC based on alignment and race.
Add horses to outdoors areas.
Redesign the NPCs with later content (classes, spells, etc.) and tweak the story some.
Redo the crafting with the later content.
Modify the start, the academy should have different training per class, not the generic 4 types.
Look into redoing the artwork on the tilesets,
Just for starters.