It seems to me that most of the problems with prisons can be solved by eliminating contact between prisoners.
A cell, a treadmill, some books and movies, and Skype privileged with your family on the outside. That's all you need. It's constraining enough to still be a punishment that you want to avoid, but it doesn't send you crazy like traditional solitary (which is more like sensory deprivation). You don't make contacts with other criminals, you don't risk getting shanked, and you sit out your sentence in an annoying yet humane state of boredom, like a waiting room.
I've thought this too. They say prison "breaks you down" and the contact with other ruffians "hardens you", well, maybe if they structured prison the opposite, and give them a ladder to climb up. So that through years of operant conditioning, an inmate can go from a tiny cell and many hours of lockdown, and gradually earn those privileges by not misbehaving, getting a GED, mentoring new prisoners, etc etc. Earn privileges all the way up to better food, having a pet, limited internet access, music, free college education, family visitation , etc. You could probably get true rehabilitation by "breaking them up" instead of down.
Isolated, perhaps, but safe? That's a whole different story.