...correct taillights, correct steering wheel, new / correct/ bumper guards (they can be add-ons like the ones that came in the Revell '69 Dart kit), and maybe a new Air Cleaner with Lean Burn computer hanging off the side (that would be truly boss!)? If they're going to enhance the kit with some new pieces, then they should go all the way!
Full wheelcovers as an option would be very nice, but not truly necessary. '78-style 6 hole "Cop Car" wheels with hub detail for under the hubcaps would be great for builders making "old" city units!
Lastly (and perhaps this is going too far), maybe some add on pieces for the engine room to "fill the gaps" between the outer and inner fenders, to give a more correct look, and maybe some of the underhood bracing that those cars had, too?