The real barrier to making realistic wargames in my opinion is the combat itself.
If you watch footage of either of the current wars in Iraq or Afghanistan you will be stuck by how soldiers almost never ever actually shoot at a target they can see directly. Most of the time they are piling fire at a bush or building they suspect might possibly be hiding an enemy 500-600 yards away. Actual visual contact with the enemy is almost never made. The most they see of them is usually when a patrol maybe moves into the area a few hours later and finds a couple of dead guys in a ditch or hut.
Obviously urban combat is different but the truth is real people value their lives far far too much to ever visually expose themselves to the enemy properly, especially close up. Real combat is much more grey, confusing, stratchy and "unsatisfying" than running into a building filled with 15 guys and shooting them within 10 meters of each other.
You might say it would be impossible to make a fun game whilst denying the player ever seeing his enemy or the satisfaction gunning him down close up, but personally I think it may be worth exploring games that try and replicate the chaos and confusion of real combat. Having a game filled with with characters that display some actual sense of self preservation might really change things. Maybe the player will derive a sense of fun not only from what he does and the direct consequences of his actions (shooting people, people getting shot) but simply from his sense of place and being involved in something more real and incredible. The first time I see a game without a magic hud displaying your mission is xyz displaying the number of flak88's i've yet to single handedly dynamite, or some other varient of this, I will rejoice.