I am considering a BBL. I want to get a thong lift. Would that correct my saggy ness? (photo)

Thanks for your question. You appear to be a good candidate for a brazilian butt lift. When planning a Brazilian butt lift, patients can look forward to a double benefit: reducing areas of unwanted fat while gaining a perkier, rounded posterior.  Each procedure must be planned according to the individual needs of patients, harvesting fat from target areas of the torso, thighs or knees and transferring it to create smooth, attractive contours optimized for each person.
That said, you definitely have to choose your surgeon correctly and make sure that he/she has the training and experience to meet your goals with absolute safety.  Best is a Board Certified Plastic Surgeon with specialized expertise in this procedure.

Hello dear!
Thanks for the question and provided information as well.
Based on your pictures, you are a good candidate for liposuction with moderate BBL, and I think you'll get very good results. 
Liposuction allows your plastic surgeon to transform your entire shape. You can improve your abdomen, hips, back and flanks, and of course your buttock, to give you a beautiful heart shape.
In general, you need to be physically healthy and at a stable weight, have realistic expectations, be a non-smoker and your BMI needs to be 30 or less.
l recommend you to make an appointment with a board certified plastic surgeon to talk about your goals and anything you need to know.
Good luck :)