One very nice feature from PzC and Decisive Battles is the option to load a graphic file as a background when you are creating the map.
That would be really helpful
First, I like this game and now prefer its more historical approach to Panzer Corps' more 'free' campaign versions.
There are room for improvement of course, here's what I've gathered so far.
Graphics:
Each equipment type should have its own, historically correct graphic. I think therer are too many equipment types sharing the same graphic. This game has the potential to recruit new gamers and graphics is often important for new players.
For instance, the various Bf109 aircraft could just have different markings to make them stand out.
Please provide an option for removing the hex grid. I think the map would look much better without.
More alternative terrain tiles. Also the tiles look a bit like blobs with not enough "interaction" between neighbouring tiles (terrain flowing over into the next hex).
Better contrast between text/unit and backgrounds.
An option to turn off the fade in/out of available movement hexes. I find it mildly annoying with too much graphic fluff fighting for the player's attention.
An option to play on a 'flat' map without the 3D tilt at close zooms. I do not mind the units tilting, but it is often hard to grab an aircraft. You really have to look on which hex the aircraft's shadow is.
Sounds: Each units should have its own sound. Having different sounds for a fighter like a Bf110, a bomber like a He11 and the typical diving sound of a Ju87 creates a better game atmosphere. There are lots of individual sound bits available, ask VPaulus

Scenarios: Every scenario should have at least one airfield for both sides, or the option to have an engineer unit build one (it might use several turns). With the fast-moving German forces an airfield was often just that, a field. There are at least one scenario (Tallin?) where the Soviets have 4 air units and there are no German airfields.
An option to keep on playing after you have reached the scenario's goal (and the scenario ends). Sometimes I would like to see if I can reach all objectives and/or destroy all enemy units before time's up.
Units: The markers for movement/attack are too small to be really usable zoomed out on a large monitor. Also, the movement marker is still green after you have moved if there's an (unusable) fraction left.
Increase the aircraft range to 1/3 of their historical operational range, and lower the ammo.
I would prefer an air unit to reach most of the map, use all its ammo and return to base the next turn. Maybe ammo should be automatically renewed (but inceasing strength should cost resources).
Modding:
Open the game up for graphic/sound mods and also options to tinker with the game values (movement, max ammo/strength etc).
That's all for now.
Erik