Well i would say a little weather editor, so you can create cloud cover and fronts of rain etc, alos if you ave hail it can *tink* on your helmet rendering your hearing useless!
Ice etc, harder and softer ground, tanks getting bogged down a bit so they go slower over mud.
A few more polygons on the weapons, makethem look nice please. the new m16 that came with suchey's marines looks just like the real thing! A bit more specialisation of the soldier classes, radio man, recon specialist, CSAR pararescue guy etc.
Lot's of realism options and animations, like you can only communicate by radio if you have one and you can only contact the base if you have a radio operator or the radio off him if he dies. Some more realistic flight models for helicopters including things like vortexes and retreating blade stall, all with options so they can be simplistic.
Give the AI more self preservation qualities, like surrendering to overwhelming forces, all scalable so you can have less determined soldiers etc.
a bit more ground cover, make fields with very low but dense cover so you have to crawl across without being seen. find a way so the woods have trees that are destroyable, so you could blow a LZ in the middle of a forest.
Divide the campaigns into the different roles, so instead of continuously switching just play the infantry campaign where you have checkpoint duty and patrols through towns where you have to look for suspicous people, and you can arrest anyone. Maybe have a few more facial expressions - this setmimic "suspicous"
If you have a equivalent to Sam Nichols have a bigger escape and evasion mission for the downed pilot with maybe CSAR, you have to use the stars, commandeer a radio maybe to call for extraction or just run for the border.
Make the enemy just as righteous as the Americans (no evil Guba evil evil person etc.), so if you have an 'opposing force' campaign you don't feel like you have to defect.
Take a look at IL2 Forgotten Battles, because it's a great game, but also becasue it's got a divided roles campaign and the crashes are superb, i hate when i do an autorotation and come in at twenty knots on a good angle of attack and i blow because the OFP engine instantly assumes speed = death.
Have an animation for everything, taking a piss, using a radio, checking a map, saluting then giving your officer the finger etc. Just to add to the realism of soldier's life.