Open editor and place your character :
Go to prone position and look around while aiming (not with freelook that only move the head up), your body will bend in a unrealistic, impossible, ugly and immersion killer way.
You can also notice that this way prevent more than 45° aim up angle.
If you use your mouse to aim left or right you will notice an unrealistic and impossible ground rotation that the character do, bot also do the same thing, their is no animation for them and depending of the rotating speed the animation is totally not matching with the rotation.

Like a have describe no more ground rotation with mouse, rework for ground (prone) stances and recycling of actual animations/stance for this idea.

Using Prone stance left or right make our body rotate like the mouse actually do but in a single rotation speed available, it will avoid desynchronization between rotation speed and animation and don't need new animation and it limit unrealistic prone ground rotation and avoid to get out of the cover just cause our body rotate itself.

For the way the ground rotation can work :
Reference video (same link again)
http://www.youtube.com/watch?v=PTfm80k_ThM

Default prone & aim forward = actual prone stance, nothing change except that when we aim left or right our body don't rotate but our arm move, exactly like the Aiming Deadzone do (Options -> Game -> General -> Aiming Deadzone), i talk about the fact to aim not directly where our general body position point but the aiming point need to stay center on the screen and with hand moving rather than upper body side.

When aiming more left the game can use the actual prone stance left same for right.
http://www.osagecombatives.com/photos/Training-Photos/IMAG0179.jpg
http://www.edmontonjournal.com/cms/binary/8397689.jpg

When looking opposite to our general body direction (aiming at our feet) the character can use sitting and/or supine position, the best will be while aiming at 6 O'clock and look down the character use sitting position like the high prone stance, and while aiming higher he go to supine position.
http://www.ssbpcrc.co.uk/html/Pictures/2011/Long%20Range/May11LR10.jpg (transition from side stance to supine stance)
http://www.thefirearmblog.com/blog/wp-content/uploads/2012/10/shotgun_supine-tfb.jpg
http://cloud.steampowered.com/ugc/560970355504977031/492CE76945465264342E5393A0C44DB154773976/ Actually in game supine position
And use of actual ingame sitting position when look down http://i.imgur.com/9zxaiNL.jpg

While aiming up a transition between max prone stance to lateral stance that permit to look up with a correct angle and finally supine position that permit to make a smooth transition between prone to stance and able to simply look backward.
Like this http://warriortalknews.typepad.com/.a/6a0133ec985af6970b016304f9923e970d-500wi
Also the lower prone stance is perfect for transition between prone to supine while looking directly ahead.

It need a fluid and automatic transition between all actually existing stance while aiming around, it mean that we must be able to look 90° up and possibility to make a transition between left and right by passing on prone or supine position depending of where we aim (forward or backward)
It provide 180° of freedom, more realism and more immersion, better look and fix several issue at once and lets prone up and lower stance free for something else.
Actual prone system (+ some bugs) http://www.youtube.com/watch?v=p5b3zWeXxvQ first link again.