As a real life serving sniper, I feel somewhat let down by the sniper experience in ArmA 2, here's my wishlist:
1)- Ballistics - Now that you have the PhsyX engine at your disposal, I would very much like to have fully simulated bullet drops.
2)- Wind - I know that this would be very hard to implement since you can't actually feel the wind in-game, but perhaps at least making the trees and grass sway according to the wind would be enough.
3)- AI Spotter - An AI that would call out targets, best estimate ranges (or if he has laser-rangefinder, then real ranges) and corrections for missed rounds.
4)- More variety - I would love to see the .338 round added in-game, maybe even the .416?
5)- Moving flora - The ability to move local vegetation to make yourself an ideal sniper hide.