This is not going to be popular, but I'd like to see Heavies trade health for front armour (and a subsequent nerf to repair rate to prevent faster repairs) in order to make flanking much more viable and lethal.
The problem there is unless you start getting to insane armour values, all you achieve is cementing tank destroyers as the only counter.
I don't know if this is even within the capabilities of the mod tools, but what if instead of more frontal armour, they had frontal damage reduction?
Say, they've got 720 HP like a premium medium, but 50% DR on the front, meaning attacking one head on gives it 1440 effective hit points (the same as a Churchill?).
That'd make them more effective at the spearheads they're designed for, but also much more rewarding to flank.