You are mostly correct about the "no new features" in Modules. However, if you go back and look at my posts where you see me saying this you'll see I made three exceptions:
1. Weather related necessary for the Module's setting
2. Terrain related necessary for the Module's setting
3. Unit features necessary for the Module's forces
Think about it this way. Can we have have a battle in January without winter uniforms, textures, and weather effects? No. So to have a Module in that timeframe we must include these features even if no other game before it has such things.
Normandy, on the other hand, does not have battles in January, so these new features are "useless" for it.
Normandy does not have AAA vehicles, so having an Upgrade that supports AAA stuff doesn't do anything for the Normandy player.
So what would a new version of the game engine for Normandy have in it that was useful? Nothing. Now, if you guys want to pay us $10 for nothing... well... that's a nice thought, but I don't think that's what you want to do
With that said, when Arnhem comes out there will be a similar bunch of unique things for Normandy just like Gustav provides for Italy. Some of them are the same. Which means when Arnhem comes out you can get some of the features that Gustav offers and some that Gustav doesn't.
Engine upgrades are designed for things like new user interface, new basic gameplay improvements (like improved TacAI, Editor features, etc.), basic changes to 3D effects, etc. It doesn't include new time frames, new units, new uniforms, etc.
I hope that clears things up a bit!
Steve