Are there interesting timelines in which Cologne stays something like the primate city of Germany? It was the biggest city in the Holy Roman Empire until well into the 16th century. It was too far inland to ever be a maritime entrepot like Antwerp or Amsterdam - IIRC the main competitors to Amsterdam around the time of the Sack of Antwerp were Hamburg, London, and Bremen - but it could plausibly be a main city for intra-German trade up the Rhine.