As I've briefly mentioned - introducing new attributes to the game would be interesting.
Projectile impacts and tracered ricochets with full collision effects...
IE if you're hit by ricochet there is damage which makes for interesting play when the enemy are behind cover.
More Command and Control of the AI Teams would open up more mapping possibilities.
Create objectives, change weapon load-outs, add pickups whilst in-game would extend game-play enormously.
EDIT:
Last but not least - scale up the engine and raise the number of objects you can have in a map.