Most of Modern I would leave alone - classes, talent trees, etc.
Things I would want changed/fixed would include:
Burst fire and Autofire rules
Nonlethal damage changed to D&D 3.5 version
Armor as DR
Recoil rules for firearms.
Nice to haves:
Additional talent trees for the base classes.
Resolution mechanics similar to Spycraft's Dramatic Conflicts.
Vehicle/starship construction rules
Azulor
Things I would want changed/fixed would include:
Burst fire and Autofire rules
Nonlethal damage changed to D&D 3.5 version
Armor as DR
Recoil rules for firearms.
Nice to haves:
Additional talent trees for the base classes.
Resolution mechanics similar to Spycraft's Dramatic Conflicts.
Vehicle/starship construction rules
Azulor