Since I like modeling, and do it more then actual gaming, I'm excited about your idea of add ons to modify the wide range of available 1/72 figures. Below are some ideas for customizable pieces and some bits that could be useful for gaming.
1. For heads: If you are going to do generic heads in sets of five, I'd recommend and would love to get for conversions.
1.a. A 'bareheaded' set suitable for US Marines/Rangers with short hair (high and tights), and mix of European/latino and African Features. Radio headsets on 'some.'
1.b. A set of SOF guys with longer hair, 'combat beards,' oakley sunglasses and headsets.
1.c. A set of African rebels/bandits (think Liberian Civil War or Lords resistance Army) with mix of bare-heads, bandanna and maybe a ball cap or two.
2. for arms.
2.a. In 1/72 I think there would be lots of generic use for sleeved 'arms' holding, metal detector (would be very useful for Iraq/Afghanistan). IV bag (for Corpsman), maps, machete, ammo cans.
2.b. For more modern wars: a "GPS/smartphone/PDA (at 1/72 scale I think they are same?), a 'toughbook drone/robot controller, small tactical radios (walky talkys), modern assault ladders, camera and or video camera, chainsaw, laser dazzlers/illuminator. 'ballistic police shields.'
2.c. For something different arms handing out a soccer ball or teddy bears, holding a canteen cup, or water (or other) bottle, or holding a phrase book.
3. Small scale bits that I'd love to see:
3.a. Bomb detection/clearance robots. (Packbot 310 or Talon IIIB, google will bring up photos).
3.b. Portable satellite dish.
3.c. Modern US/UK 81mm mortar (google M252 81mm)
3.d. Generic stack of ammo cans (could represent insurgent cache or friendly ammo resupply)
3.e. Small (~3KW) generator (s) for COCs.