with targets invisible in the "blue" dimension, it is impossible to judge distance (without moving and triangulation, you can't tell if an enemy is just under your nose or on the other side of the battle). therefore, i suggest there be an indicator of distance.
such an indicator would also help in the correct choice of weapons for far away targets.
indication could be given numerically or visually (bars? 'nother dot?). integrating it in the aiming circle would give ultimate positioning information, but might confuse new players.
do you think this could be done?