Currently the prison is good but it should be expanded as only one subject is needed to clear out the entire prison. Either that or increase the respawn times in there.
Now for the additions, there should be more objectives for subjects to do like taking orders to assemble certain guns, or certain armors, where certain divisional combatics/weapons would be rarer to get for orders, but takes more time. And to expand on that maybe have certain ADHP ranks can do certain tasks due to them being more trusted. Maybe add something like putting ammo into clips for AEGIS. Just add more for Subjects to do rather than just mine salt.
tldr; Add more tasks for Subjects, expand prison mining stuff or make respawn times in Prison to be more quick.

