Not any old X will do = X needs to be very special.
I am looking for a new dress, but not any old dress will do. It must be a special dress. I must feel like a princess when I wear it, with a pretty color and a flattering fit, and just a bit of sexiness but nothing trashy.
My daughter wants a new bike, but not any old bike! She wants a ten-speed, with a comfort seat, and a rack for her water bottle, and a place to store stuff while she rides. (Good thing she's paying for it herself.)
As we try to picture the future, not just any old future will do. The future we envision must be... [whatever it is the author thinks it must be].
Student or Learner