Having to go prone to fire with M240 is that by design and has been like that since last summer. The weapon has such an enormous power compared to the assault rifles that it has been made significantly less agile to use for the counterbalance.
Less agility also means that when fighting against an M240, you can more easily evade the fire sideways as the M240 field of fire is limited, and you have more time to act when flanking an M240 which is directing its fire elsewhere.
You don't specifically have to stand up to change the direction of fire with M240, getting to the crouch stance is enough. Truth to be told, it doesn't make any difference in speed though, as there's no transition penalty between standing up and crouching

Prone movement and rotation will be added in the future, and when carrying an M240 the speed of movement on prone will likely be set lower than with other weapons.
Reloading time is currently the same, but it should be a lot greater for M240.
Anyway, your proposal can be made to fit in some ways I think, without losing the clear difference in gameplay style M240 implies over the other weapons. For one, the accuracy could be made ridiculously low when shooting standing up and the shooting pose should actually be shooting from hip. Second, while standing still with M240, the pose should be such that the weapon is simply carried and as such not immediately ready to be fired with, indicating a short delay to start the fire, as the handling of the weapon should be clumsy due to the weight. Third, I would keep the shooting stationary, even if standing up.