>>2044414These plots
>>2044416 along the MassPike are already develop-able, and have been since before the Big Dig started. If I remember right, there are something like 25 parcels starting from where Commonwealth Ave passes over the Pike, going all the way to where Harrison goes over before the Fort Point tunnel. The Hynes Convention Center, a portion of the Prudential Center (but not the tower), and the new Car Gurus headquarters all used some of those parcels.
The problem is that despite the air rights being available and fairly cheap, the construction and engineering costs are way higher, and it has only been recently that the cost of land elsewhere in the region has made these parcels viable. As I understand it now, there are at least two additional projects filling in sections near Fenway/BU. However, it will probably be decades before the whole thing is filled in - it'd likely take a concerted effort from the city with tens of billions in funding to get it done in my lifetime.