Leaning isnt used in any online multiplayer game(or none that i know of), and there is a good reason for it. Imagine the way you would feel aiming at crouching guy who is leaning left, his model would be..meh, corrupted. Even now its hard to hit someone when he is crouching.
About the prone, as jitspoe said it doesnt fit this game well. Even crouch is only used to dodge lines/peak over the corner without using walk/run. And i didnt even mention new paths which werent supposed to be there this would open (maps which come to mind are shz33, pbcup_renoir, carpathian,...).
Imo, not the a good idea for this game.