Things I would like to see:
-More details, just like in HPP or BICE mods.
-More ahistorical events, to make it more interesting to play different ways (US on their own, Japan in the Allies, China in the Axis, German successful "Sea Lion" & Kriegsmarine focus)
-More pronunciation on economic aspects of war (oil/steel/tungsten and food where massive problems in the main theater of war, Europe. This would stress the importance of strategical areas with resources like the middle east etc.)
-A complete overhaul of the supply network and a possibility to manipulate it for special needs of the player. For example the possibility to create supply depots and route supply between them. Possibility to create railroads (with an own map mode) between positions and allow better shipping of troops and supply.
-An overhaul of the non-land-warfare to make naval and air combat more important and interesting. It should start with more details, specializes OOB's etc.
-A complete overhaul of the AI to make the game more challenging. AI should make effective use of different units like Paratroopers, Mountaineers as well as effective use of the different types of planes and ships. It would be interesting to teach the AI to make use of naval invasions, to breakthrough etc. It would be interesting to give the AI the possibility to focus on more diverse strategies. Spoken from Germany-Players-view: fighting a UK/France fully focused on land warfare and lacking heavy investments in naval buildup would force you to adapt: efficient spying to see what the enemy is doing and get a reward for a good adjustment on your own. To beat the UK's expeditionary army on the continent should make a "Battle of Britain" or "Sea Lion" easier, as a satisfaction for a harder "Fall of France".
-I don't have a good idea to offer for this, but I would like to mention it anyway: introduction of way to ordered retreat. Needs to be carefully balanced to still allow breakthroughs and encirclements.
-More importance to minor/major allies of the factions. The AI ruins any approach to play "coop" with the AI partners. For example, it's quite rewarding for Germany to conquer a lot more than historical, as the AI units were weak and stupidly used. It's fine if you want to go on your own, but if you want to attract more allies to join your war effort, it should be rewarding, too. Perhaps it should be something like a penalty/bonus for combat efficiency, to simulate the fact that the enemy is facing different enemies with different equipment and tactics. Or to just boost the AI to a certain percent for every new puppet/ally you can drag to your faction. To give it event based OOB's and a more accurate and faster reaction to the "province of interest" set by the player. I saw Italy trying to invade Malta with a small force of unprotected Transports and no bombing/coastal bombardment eg.
-A balance adjustment and "new content" in the way of long going wars past 44/45. Most of the games are over much earlier and balance late war isn't given. Supply system, nukes, the inflexible diplomatic, new technologies like rockets and jet planes: these things aren't working good or aren't really jumps in warfare.
-Better presentation of the game. I like the graphics of EUIV, and an evolution of it with (please smaller) sprites for all the units to zoom in and watch a fight going on would be great. Tactical symbols on overview, sprites on close up, that would be fine. I would love to see the pride of my fleet steaming through the stormy see and firing their heavy artillery. I would like to see my interceptors hunting down enemy bombers and I would like to see the factories or streets ruined when they are bombed to the ground. I'm not talking about FPM graphics, but at least the level of Civilization V (have a look at the RED WWII MOD for an picture of what I mean) would be nice, though still 5 years behind the time.
-A more accessible, more comfortable UI. Have you ever reorganized the Red Army? It's a pain. A better way to sort officers. A better way to show HQ or radar ranges (like the plane mission range). A possibility to retreat/maneuver in units shown combat window. A possibility to auto adjust sliders in more than one focus. A "are you sure" question if you accidentally maneuver units that have nearly reached the next province eg. or planes that are rebased in paused mode to the wrong airfield and can't be redirected.