>If Hitler quit while he was ahead and consolidated his positions instead of trying to expand further, no north africa campaign,
Well, for starters, there'd still need to be a North Africa campaign. The British were on the offensive originally there, and all the long term advantages in terms of harbor space, railroad capacity, ability to project troops there, etc. favor the British. Rommel's initial orders weren't to go conquer Egypt, it was to prevent imminent collapse of Italian forces in Cyrenica. If you don't attack the British, they'll attack you.
>Would he have economically struggled
Probably, but that's almost a given; the Nazi economy was very badly mismanaged.
>Would Stalin eventually invade?
Probably not, not unless it was looking like the Nazis were about to collapse on their own. Stalin was, first and foremost, a cautious opportunist. He liked his wars short, and against tiny, diplomatically isolated countries. He's not about to start a war against a power of near equal size and with quite a few advantages in their bag.
> Would Britain or America attempt any naval invasion of mainland Europe?
Not immediately, maybe not ever. Much more likely is that Britain would keep the war up on a low intensity level more or less forever. They have a larger economy, and an air doctrine that's far more effective for that sort of strategic bombing war than the German CAS doctrine. And the U.S. was getting more pro-Allied and anti-German by the month after France fell. Sooner or later, they'd pile in.
That being said, if there's no Eastern Front, you're probably not ever going to see an invasion of mainland Europe; too many German troops and too easy for them to reinforce wherever you landed.
Much more likely is an attempt to win the war by airpower alone, and doubling down on that, as well as overrunning of tertiary theaters like North Africa where it would be hard for the Axis to meaningfully project force.
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