I'm sorry, but I really don't like this idea at all. Some of us have enough trouble getting into the right position in a fight as it is (AoE shapes, being within range to hit, being able to see your own character when the mob is 10x as big as you are, etc.). Adding this kind of mechanic only makes things more difficult, and not fun. If I'm acting as healer in a party, I don't want to have to spend the entire battle making sure I'm in exactly the right spot to not get hit as soon as the mob realises I'm squishy. Likewise, as a tank, I don't want to have to be running around in circles, trying to stay between the mob and the squishies behind me so they don't get taken out in one hit. There's also those mobs that have the knock-back skills; the minute they knock the tank out of the way, any squishies who happened to be behind the tank are now next in line, and chances are good they're going down quick, especially considering how long it takes for a player to get back into position once they've been knocked back that far.
In another game, this could possibly work, perhaps if it was implemented this way from the very beginning. In this game? No, thanks.