I briefly covered some German superguns before, but the topic is an endless source of amusement, so I present another one: a 150 mm gun for the VK 70.01 (Lowe).
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Just as the Soviets all but gave up on putting the 152 mm Br-2 gun into a self propelled gun, the Germans came up with the idea of putting an even bigger gun (although the extra length didn't grant it must better characteristics) into a fully rotating turret. Of course, we know how this story ends: eventually the laws of physics and common sense won and this idea, like dozens of other German wonder-weapons, remained on paper.
Edit: my loyal reader Critical Mass helpfully comments that the VK70.01 was cancelled in favour of an even less practical tank, the Maus.