Well the central part of the League as we know it depended on Lubeck and the cities having the "Law of Lubeck" I think, plus the merchants on Visby which IIRC were first "patronized" by Henry the Lion specifically as owner of Lubeck.
But yes, I guess Hamburg and Cologne could themselves have formed a League; Hamburg was big by contemporary standards and was definitely a center of trade after the decline of Hedeby, while Cologne was the largest city in Germany for centuries and the center of the Rhenish trade with England and the North...
However, the desire to form city leagues in Germany for mutual protection was a direct result of the decline of the Emperor's power in Germany, and so a feature of the LATER Middle Ages, not of 1066-1250.
So Zhai, after reading your other posts, I think if we both have our way, it will never reach that point! I plan to be dominating the Baltic as HRE way before the mid-13th c., so there wont be a need for the League in my campaign...