I'm a huge fan of putting a market here. In fact, do it up right--make it a great civic place, built to last out of solid granite, in a classic, Greek revival style. Have a great central corridor with booths selling all sorts of foods and wares, and in good weather have stalls lining the outside. Wait, come to think of it ... that would just get turned into a mall food court after a while.
In all seriousness, a market building modeled on Eastern Market in DC, filled with a mix of permanent vendors, such as bakers, butchers, fishmongers, a simple cafe or two, with outside stalls on market days selling flowers, produce, art, questionable tobacco-related paraphenalia, what have you, on weekend market days, would be great, and it would draw real shoppers and tourists alike. The hordes marching to North Station would stop here to pick up food on their way home. This should not rule out a market at South Station, too.