maybe i'm not understanding your idea properly, but since ranged classes do not have a minimum range, it seems to me like your idea would just cause a raid to stack everyone inside. unless you are talking about the damage being similar close to the boss as it is now... in which case i think people would probably still stack on the boss.
You see the problem as it stands currently is that being in Melee for various reasons causes more problems with DPS and Healing than just standing away from the fight hitting from range, therefore you automatically put the ENTIRE group at a negative placement if it just has Melee players to work with. Working on Nightmare modes needing to stack ranged DPS is VERY BAD DESIGN.
The idea here would be to give a positive to balance the negative from being in melee