This could of been said before from previous posters.
- Ground and underwater knifing animations for stealth operations.
- To improve thunder, aircraft sounds and other certain sounds that overwrites or dominate smaller sounds that can affect NPC AI in hearing.
eg. sound of thunder while being engaged by single gunfire at the same time (can be very useful for stealth operations when the timing is right).
- Rappelling option for entering buildings, mountaineering and Helicopters for quick descent.
- Visible weapon smokes upon firing with barrel heat for infrared detection FLIR.
- Feature to hack other radio frequencies and listen to some nice random NPC dialogue.
- Improve enemy AI actions to eat, drink, train, group scout randomly, interact with NPC's with random dialogue and interact with objects in a unaware situation.
- Armor piercing bullets for powerful sniper rifles to give it the ability to take out personnel inside heavy armored vehicles.
- Improve animals AI to make them eat, drink, hunt and move in herds while being chased or threaten by loud sound.
- Real time weather to make graphical/dynamic changes to the environment (I don't think any game has this feature).
- Lastly, it will be pretty cool if commander/construction modes can be combined to feature more like real time strategy games to give it a FPS and RTS feel (to give the player a sense of both enjoyable experiences).