One solution to this could be a change to the point where the attacking tank aims (however, I only presume it is in the middle of the tank in the receiving end).
The point could be moved to the (middle of the) rear end of the tank; what would it mean?
-> Attacking from the front gains you frontal hits
-> Attacking from the rear gains you hits to the rear
-> Attacking from the sides gains you more rear hits than frontal hits thus increasing the effectiveness of flanking while still keeping the same armour system.
A crappy image to illustrate the point: