A few things would differentiate this game mode, starting with:
Change in the Damage/Kill Mechanic
Many players have discussed the idea of a "hardcore" game mode being implemented into this game, and this seems like a good opportunity.
One of the oldest ideas I've seen is to implement it where a tank is only destroyed if all its modules are knocked out, but I personally dislike this one. What I imagine instead is that yes, module damage should be more severe than it is in the current game. If you find yourself tracked, it isn't just going to repair itself....and a fire isn't something you could easily recover from.
Instead, what I imagine is the idea that your tank's durability is MASSIVELY reduced. Now, instead of doing HP damage (which isn't very realistic), a specific gun might have a damage ticket. Certain tanks have a certain number of damage tickets, such that some lighter weapons may need 2 or 3 hits to kill and a larger one might only need 1 penetration.
The durability of a tank may depend on the scenario. Let's say you were doing a "Fall of France" scenario. The British 2 Pounder may need 2 hits on a Panzer III to secure a kill, while the 75mm KwK 37 L/34 on a Panzer IV could knock out most tanks in a single hit.
This does lead to two questions of course. The first is "what about HE damage?" To that.....I'm not sure yet.
The second question is "well, how would you balance certain vehicles?"
How Tanks Are Handled
This is the really central idea to this game mode.
You see, the way I see this one thing that affects the realism of certain vehicles is that they aren't exactly historical accurate with regards to what they're armed with. Take the Ferdinand. It wasn't ever armed with anything bigger than an 88mm....but gets a 12.8cm gun. Or the Tiger tank getting the 88mm L/71.
In game these decisions make sense. They give the tanks a rough approximation of their actual performance relative to their tier.
HOWEVER, what if tiers weren't the primary concern? The Ferdinand wasn't fighting things like the IS-3, the Pershing, or the KV-4: its primary opposition would have been things like the T-34. At that point, the 88mm gun makes more sense, and is roughly as powerful against something like the T-34 is as the 12.8cm is against a Tier 8 tank.
This also has an interesting application that tanks that were historically designed to be powerful due to their armor (like the B1) actually can use it effectively.
Here's an example of where limiting tanks to historical specs can be balanced, using the Tiger-H as an example. The scenario would be an Eastern Front, mid-War:
In historical configuration you'll get an 88mm KwK36 L/56. This gives it fairly good accuracy and penetration when you consider what it will be fighting: the T-34, the Su-85 and Su-100, KV-1, KV-2, and BT-7. Furthermore, the armor of the Tiger tank would be superior to anything the Soviets could field, being able to deflect shots easily from all except the KV-2 and the Su-100 (remember, armaments would be historical, so the Su-85 would have the first 85mm with 120mm of penetration, and the KV-1 would have a 76mm gun.
To balance things out, the Tiger would get the Maybach HL 230 P45 engine (not the top one), limiting its power to weight ratio. Also, some other stats might have to be balanced around, such as the turret traverse speed.
Obviously, this may sound a little overpowered, which brings up the final bit of my idea:
I have a couple of scenario ideas, one of which I've come quite a ways towards solidifying (I'll give an example of it farther down).
How this would work is that you have two teams, with certain tanks allowed on each side. Tanks have a set configuration for the specific scenario, AND some tanks have a unit cap. Such tanks would be things like the Ferdinand for a Battle of Kursk Scenario. The reason for this is that relative to what it's fighting it's VERY powerful, and that it really was in limited numbers for that battle. This is an example of balance built into the game, where we don't want certain vehicle dominating the game.
Also, because certain tanks underwent upgrades throughout the war you might find in one scenario a tank feels "gimped". An example would be the Panzer IV; it was present during the Battle of France, but not with the same gun we see at the end of the war, and with a different turret. The British tanks are a great example of this.
Scenarios would also take place on maps that best typify their purpose. A Fall of Berlin scenario might happen on maps like Sigfried Line or Himmelsdorf rather than Serene Coast.
Most interesting is that certain scenarios may not have to be strictly "Germany" vs "Everyone Else".....can you say Cold War, anyone?
Perhaps the best way to get this all across is to make a few examples, and give my rationale for why things are the way they are:
Fall of France
I shall finish this later, with a few more scenarios to include and more elaboration on the one I've already got, but I can't finish it right now. Read over it and tell me what you think. Some things can be refined here and there, but this is my rough idea of what I'm thinking of.