here's my deal...
All the black holes in the universe collect and compress matter. This compressed matter is deposited in another dimension/alt universe.
If the black hole is large enough, eventually the compressed matter reaches a critical mass and voila, big bang, new universe, over and over, billions and billions of universes.
As for thinking the expansion would be spherical. Since the laws of physics did not apply at the time of the BB, can that be assumed?
What are the properties of the shell you are proposing? I am not getting the picture. If it is what I think you mean, it would be like a Dyson sphere surrounding the universe. Maybe we could eventually colonize the inside of the sphere.