Two problems with that premise:
1. how do you define "nearly impossible"? If you don't really have a chance to beat the US as Germany, then how much fun can you have? Do you play until the game ends and get your satisfaction from a victory on points???
2. with that set-up you proposed, how can you play the Allies? Wouldn't they be a complete cakewalk? You can't have weak Axis, strong Allies and have a challenge with both sides unless you handicap the player regardless of his chosen nation. That would bring us back to difficulty settings which usually handle handicaps.
1) Winning as Germany doesn't mean the Axis have to conquer the world (indeed, shouldn't!) I would hope that at some stage, the HoI diplomatic model is nuanced enough to recognise that a Germany that's conquered Eurasia would be able to negotiate a compromise peace. I'd personally like a model where the Allies could run out of puff on the way into Germany, and be forced to negotiate a peace as well, but one thing at a time.
2) The allies could and should be a challenge early (the Brits didn't start winning much until late 1942), and then after that they very much should have the advantage if the US is involved. The most challenging gameplay moments I've had playing 'historical' games of HoI3 have been as the Allies, not the Axis (the Allies start hard and get easy, the Axis just start easy). The challenge is to win quicker than was the case historically, but also if the USSR falls to Germany, then getting into Europe at all, should be really, really tough (easier than invading the US, sure, but still really, really difficult), and then there's the hope of a negotiated peace again.
Ideally, I'd like both sides to be more challenging to win with than they were in HoI3, and I reckon there's a fair chance this'll be achievable, just through improvements in both AI, and in game mechanics that should mean the AI has an easier time working with them.
As per my earlier post, I don't mind the vanilla game catering to people that need black-and-white 'win/lose' conditions rather than something more historical and nuanced, but I personally prefer something a bit deeper.
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