I think the road is endless with various campaign and scenario options.
I'd even like to entertain the idea of starting as a country of YOUR choice and developing a system (campaign) that could enable you to invade whichever country you want, whenever you want. (until it's totally defeated.)
Example - Have the USA invade Canada in '39. Invade Mexico in '40.
(all the while, Euro countries are bolstering their defenses) Soon you have to trek across the seas to invade them.
The object would be: you could be one of any 15 or so countries. Pending on which country you decide to invade first, would dictate your next step. You can choose where to fight your battles, which countries to invade and in whatever order you wish to do it. (the opposing AI countries would build their defenses as time goes on and get stronger.) So if you invade a weak country first, you may sacrifice defeating a stronger country early on before they build up their strength.
It would be extensive, but offer many possibilities.
Then you could play as a tiny country ruler, and try to take over the world.
As you defeat countries, your NATIONS power gets stronger, prestige increases and amy becomes more powerful and inventive with new weapons.
If you lose battles...the war starts to move back towards your homeland and you have to play defense rather than offense. This would be a huge task for developers to undertake all the possibilities....but it makes WWII fighting new and exciting to think you can make all the decisions on where it goes and how it flows.
Just a long drawn out thought....(sorry)