I agree that thickness of the circle should be enough, combined with pulsing green/transparent or green/white. (Or maybe dark green/light green).
Something else that'd be nice are fainter (maybe about what we have now) green circles around all (unselected) friendlies (and red around enemies, blue, yellow, etc), that increase in intensity as you zoom out. When zoomed out fully, the actual models would be too small to make out properly, but coloured circles would give you a good glance at your overall tactical situation. (And throw in a hotkey to turn these circles on/off, for screenshot-takers.)