I'm pretty sure the Globe plant is built atop a landfill or some form of former waste site. Redevelopment options are quite limited, if that's true.
As for space: the paper has a bunch of space that extends all the way into the edge of Savin Hill. It's much larger than the footprint Fenway sits on, and patching the Globe, 56/Chambers and Shaw's properties together would give you plenty of room, with a short walk from the Red Line and commuter rail, and right off the highway. And except for one side (Savin Hill), you're surrounded by either industrial property, park land, a highway, or schools (BC High/UMass-Boston), few residents to deal with. Ideal in a lot of ways, and you could get a waterfront view.
I don't ever see that happening, though. I doubt the Savin Hill residents would go for people walking out of the ballpark and pissing on their lawns.