The discussion about Prussia could also be a good starting point for the alternative campaign.
Hitler is removed by coup d'etat by the Prussian military circles. Leadership is taken over by some high ranking general and/or a cicilian like Canaris. As a first measure to normalize the situation, Germany makes peace with France and hands over most of their occupied territories, so the U-Boats do not operate from French ports, and instead use the Dutch colonies of Curacao/Suriname/Indonesia. There are then still many alternatives as to who wages war with who. Even a neutral Britain would be a possibility if hostilities continue with the US / Russia or even Italy.
I would also like to see U-Boat operations from Iceland or some occupied islands north of the UK (Shetland / Faroer). Ireland and Spain are possible candidate for entry in the Axis, and a new run for African colonies offers all kinds of possibilities.
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