The German Ferdinand or better known ‘Elephant’ was indeed a powerful Tank Destroyer. I like the rule proposed above for Tank Destroyers firing during the opening fire step, at a 1:1 ratio against attacking enemy Tanks. That’s pretty good.
But what exactly are we trying to achieve in this thread? One type of unit with many different roles? As far as I am aware there were two main kinds of mechanised support units in the tank family: Tank Destroyers and Self-Propelled Artillery. The term ‘Assault Gun’ could possibly be a little ambiguous; however, I have always associated it with the Self-Propelled Artillery unit or ‘SPG’ (Self-Propelled Gun). I guess that in Axis & Allies, it would be called the ‘Mechanised Artillery’ unit, to fall in line with Infantry/Mechanised Infantry.
So as far as I can see we ought to have 3 classes - Tanks, Tank Destroyers and Self-Propelled/Mechanised Artillery/SPG units, each with their own rules. This would pretty much encompass armoured warfare.
Therefore I guess we can create rules for the two new types of units (TDs & SPGs) as they are produced by FMG and HBG. When FMG produced their Italy Combat Units, they included the Semovente 75/18, which is a SPG. What they did not produce was a Tank Destroyer unit, but hopefully this will be rectified by either company in the future.
I would have thought that both TDs and SPGs would have the Blitz rule, for starters. But I think they should otherwise differ in their capabilities.