Agree there is plenty of space there. But the sport that has the most events is basketball. And expanding the north concourse does little to help make the gameday experience for basketball fans better.
It would make a huge impact for basketball (men's and women's) and of course, would help for football and lacrosse as well.
On the west side, you could expand the lower and upper concourses, expand the locker rooms, and make entering and leaving the dome far easier via escalators and/or elevators.
Opening up the western wall to sunlight with a glass exterior wall would really make the venue feel more open (along with the huge additional open spaces). It would be a hugely popular improvement to the game experience if funds can be found to make it happen.