Questionable whether such gigantomaniac designs would come to life under other than Hitler's (ie by default more rational) leadership... I'd rather envision tank main gun stopping at 105mm, later advancing to 120mm (or around- say US and Brits going for 120, Germans for 128, Russkies going 107mm->122mm->130mm), with this caliber reserved for heavy tanks, so... 1950's, med. tanks 88/90/85/20pdr, heavies 105/120/107, later medium 105, heavies 120/128/122/130... with maybe the German 15cm KwK coming to life but for exceptional use on some übertank with minimal practical employment or some SPG with a little bit better one
If you really don't want to leave the 175mm thing, I recommend you go for 17cm as that was caliber used in German artillery already. Also it would come with bonus - the 17cm field gun had practically same lavette as 21cm howitzer IIRC so your wunderpanzer may have two versions, one with HV gun and one with destructive howitzer.