Suggestion - Replace the Wehrmacht's Flak36 with the PaK43/41. Possibly add a halftrack - for instance the Sd.Kfz.11 - that can tow the gun around and give it some limited mobility.
The PaK43/41 Why I think this is a good idea - The Flak36 is currently featured in Company Of Heroes twice, as part of the Wehr's Defensive Doctrine and as part of the PE's Luftwaffe Doctrine. Replacing the Wehrmacht Flak36 with the PaK43/41 - which has similar firepower - would make the two doctrines feel more unique. I also think that replacing the static Flak36 with the (slightly) more mobile Pak43/41 would be good for gameplay because players wouldn't end up in situations where their expensive Flak gun is far away from the action doing nothing because they can't move it.
Adding a special artillery tractor to drag the PaK43/41 around, in my opinion, would be a good decision, because it's a really heavy gun (nearly 10 000 pounds according to Wikipedia, or four times the weight of the American M1 57mm) and it would look silly for a standard anti-tank gun crew of three men to move the thing around the battlefield.
It makes more sense to replace the Wehrmacht's Flak36 with the PaK43/41 than replace the PE's Flak36, because the PE model is supposed to be part of an anti-air doctrine.
Thanks for reading.