This has been discussed many times before, but regarding Fluid Posture (as it was called back in Raven Shield), I would like to humbly suggest a few tiny tweaks that I reckon would greatly improve the system.
In RvS we had to use the mouse to fluidly adjust our posture, but since we also use the mouse to aim, it rendered the whole mechanic less useful than it potentially could have been.
In reality, practically speaking, we either lean in-and-out from a stationary position, or we slice the pie in an upright posture instead of "lean-strafing".
Therefore I think it makes sense to use a modifier (such as holding Left Ctrl) in combination with the movement keys (i.e. WASD) to fluidly adjust the degree to which we lean as well as our height/stance, and naturally, releasing the modifier key would return us to the nearest default stance.
I'm fully aware that Arma 3 has this Fluid Stance system too (as Dslyecxi calls it), however Arma is using 9 distinct "levels" of stance instead of it being truly smooth/"fluid".
In addition, Arma 3 also by default makes us step out when we press LCtrl + A/D, instead of Fluid Leaning.
What I'm suggesting here is: Keep the current system of leaning and stepping out and rolling, because that system is simple and straightforward and it works, but at the same time please at least attempt to implement the Fluid Posture system in which holding down a modifier allows us to adjust our height and leaning in a smooth and continuous manner with WASD.
Thank you for your consideration.
A couple of related vids: