"I've never seen a scenario where attacking up the middle was a good idea".
Isn't the edge problem intimately related to mission objectives and victory conditions. Maps are usually designed around the mission objective (doh!), and there's a limit how far the attacker can wander away from it without loosing it.
I have no idea what the new victory system will look like, maybe it will partially solve this.
But another idea, expanding on the Danger Zone, is to have a Twilight Zone. Units that go too far out on the flank risk loosing control, and will go missing. Either permanently, or temporaily, and in that case it may have taken damage. My thinking is, if the enemy flank units are abstracted (and "invisible"), so should their fire. Another thing, these zones should not be rectangular, but rather triangular or half-circle shaped.