Flat spots/sagging skin on butt. Could muscles fill out the holes with weight training? Any surgery that would fix it? (photo)

It is very difficult to determine the exact size and shape implant you will require to best match your ideal butt image without an examination by a board certified plastic surgeon. Not just any board certified plastic surgeon, but one with many years of frequently performing breast augmentation surgery including different approaches, techniques and implant choices. This is because several measurements not to mention your breast characteristics are needed to determine the optimal implant size to obtain your goals. Without knowing these dimensions it would be difficult to make this determination. For example, the existing base width of your buttock will determine, in many cases, the maximal volume per implant profile that you can accommodate. Liposuction of your lower back and fat transfer is also helpful as an adjunct to implants. Using “want to be” photos however are useful if simply provided to the surgeon.

Hello dear!
Thanks for the pictures and provided information as well.
Based on your photos, you are a good candidate for butt implants, to fix your problem, and I think you'll get very good results. 
l recommend you to make an in-person consultation with a board certified plastic surgeon for a proper answer and to talk about your goals and anything you need to know.
Good luck :)

Hello and thank you for your question.  I believe regular weight training especially squats and lunges would help fill out your buttock.  You appear to be younger and your skin elasticity should still be elastic and tighten up as well.