1: I would like to see an easier way of dealing with the combat on multiple fronts.
For example, you should be able to designate fronts.
If your germany, you shoudl be able to designate a Russian front and French front by selecting an "add front" button, and selecting by either choosing in a list of clicking on the map which provinces on that front.
Then whey you are deploying new units you should have an option of "deploy to XXX front". You should not be required to chose a front, you should also be able to deploy them to whatever provinces you want. Additionally, make there be an option to assign a leader, or have the AI assign a leader, to new units deployed from the force pool. This should be done when clicking on the unit to deploy it so you can manually assign them a leader as your deploying them. It would make deploying a large number of troops easier to manage.
You should be able to chose a general or two to oversee a front. This general could affect performance, such as offering more transport effectiveness for quicker movements of supplies to the front, offer leadership bonsues to his subordinates, etc.
This would help yuo micromanage your different fronts.
2: Specific units. When you research "light tank", "medium tank", "fighter", etc, you should be given designs for a specific unit. You should have a slider indicating how much "reinforcements" you want to designate to replacing lost tanks, aircraft, and artillery. Those should not be handled by just "reinforcements", reinforcements should just be manpower. Replacing equipment, such as specific aircraft models, tank models, etc, shoudl be reflected on supply lines, as well as your production capacity (have a slider to determine the rate you are trying to replace those losses).
Replacing armor, plane, and artillery loses should cost metals and cash (and any other "new" resource that makes sense).
3: Make it so that you could designate provinces/states for bombing runs, meaning if you want a general bombing campaign over an area or on a specific province.
4: Make food a resource and make supply lines more vulernable. If the population doesn't get enough food increase dissent, if the armies don't get enough food lower their morale and increase the chances of them deserting.
5: Better chain of command. Make a high ranking general in charge of a particular front (or theatre of combat), then other generals, ltns, majors, etc under that high generals command. The higher ranking officers, like the high general, should impart bonuses on everyone under them in the chain of command. The general for the particular theatre could impart bonuses on the leadership of the other generals, which would make those generals be able to impart bigger bonuses to the troops under their command.
Limit these theatre generals to particular countries, or make certain coutnries theatre generals better than others. For example, Britain and the US and Germany could have some good ones, but Italy might have crappy ones, and newly released countries would either not have any or have crappy ones.
6: Improve the covert intelligence system. Make it so that enemy generals are targetable (assassinating an enemy general in charge of a theatre might cause the necessary confusion to break through their lines, for example).
Allow the ability to disrupt research teams, and even try to recruit research teams away from countries.
7: Enable you to "steal" tech blueprints from an enemies capitol that you/your allies control. When annexing a country allow you to chose which (if any) tech teams to try to convert to working for you.
8: Allow research teams to gain experience in particular research. For example if you have someone researching nuclear physics but they do not have that specilization, if they perform enough research on tech that has nuclear physics they should gain that skill and gain in profiency as they continue to research more and more.
Perhaps re-work the research where each tech team has a skill listing of every type of research, some higher than others, and just have that level go up as they complete research relating to each research skill?