Say you had the power to change the M4 stock in any feasable way.What would it be?

I'd change it into a Vltor modstock.  They incorporate most of the changes that are practical before going to a completely different design.  I wouldn't mind a foam rubber cheek weld like the ARFX though.

I'd change it into a Mag-Pul. Other than that, I would use a Colt 4 pos setup.

I would come up with a system that mounts the stock in a pivot built into your vest/gear that would steady the weapon more, provide 100% retention and replace any need for a sling system... There are quite a few problems with this such as being able to effectively use the weapon without having the shoulder socket/mount, and also the need to transition to your offhand... But its the direction that I would take it in..