As the maximum profit is to be earned from high value goods, I think a good profit making strategy would be to establish trading centres in cities that produce pig iron, wool and meat. Adding weaving mills, if possible, will allow for maximum returns from the wool producing centers.
So, Malmo, Visby, Rostock, and Aalborg are good options for expansion.
On the other hand, if the aim is to provide for towns, with the aim of increasing reputation, then expanding to cities that produce grain, beer and fish would be a good bet. Since, Luebeck itself produces grain and fish, although at significantly higher costs, the options to expand this way are very limited. Most probably, the expansion will have to be spread across multiple cities to fulfill the need for each of these resources.
Hamburg is a cheap beer producing city, and makes for an excellent investment.