I would like to see a couple things:
1) Realistic use of body armor. In games like Rainbow 6, guys in full bomb suits(level IV), often can be put down by a short burst of 10mm from a carbine. That's not realistic. That kind of armor would harldy be phased by even AP rounds from such a weapon. Granted it still COULD kill, going through seams, hitting the exact same spot repeatedly, etc., the chances of it doing so are low.
2) A realistic fatigue system, with a burst fatigue, and then a long-term fatigue system.
3) Aiming down the gunsights, and a fixed system of bringing the weapons to bear. The weight of the gun and the amount of armor should all directly control the maximum turn speed of the character. I see too many games where guys with 50 cal Barrett M82A1s spin 180 degrees instantly and make kill shots. Won't happen in real life.
4) Realistic differences between the weapons. Don't bother balancing guns out so that they all are equal in some way. Some guns are not equal to other guns.
5) More interesting scenarios. Be able to play everything from a bunch of terrorsts/freedom fighters holed up in the woods/mountains/jungles/desert/etc. to an elite CT squad hunting down Al Queda scumbags high in the cold Afghani mountains. Flashpoint does have some things to offer in this department, but the multiplayer system is horrendous, as are the system requirements for what it does. I would seriously suggest you try the Serious Engine, as is used in Serious Sam: The Second Encounter. Though the game looks good, it doesn't even use the engine to half it's potential, so seriously check it out
Not to mention it's one of the most seriously modifiable engines out there.