Can my natural rectangular or straight waist be made smaller or into an hourglass shape with a BBL? (Photos)

Thanks for the question. 
In this regard, I recommend you waist and back Liposculpture, getting better curves especially in the waist, improving your body contour.
Finally, I suggested take the fat we get and transfer to the buttocks and hips, to mold them and get better volume, contour and butt projection, is that we know as "Brazilian Butt Lift" (BBL).
Kind regards

Hello, thank you for your question. Fat can be removed from any unwanted areas, this will depend upon your selection of surgeon and your expectations for the BBL. Fat can be harvested from the abdomen, flanks, thighs, arms, back, etc.  without creating any contour deformities and transferred to the hips and buttocks. If the waist was smaller at one time, the waist can likely be reduced with liposuction to those same measurements or smaller provided the muscular and facial integrity has remained the  same. There are a lot of women who can get nice results from the BBL despite thinking that there is not enough fat. Schedule a Skype, or email consultation with a board certified plastic surgeon who specializes in thin patients to determine available fat stores for the transfer.
 

Hello, thank you for the question and great photos.. 
After analysing your case, I can say you're a great candidate 360 lipo vaser + BBL, this would fix your problem, give you great results and improve the look of your waist, belly and your buttocks. 
It is important that you communicate to your board-certified plastic surgeon what are expecting, to make certain that your goals are realistic. It's helpful to show pictures of results you'd like, but please keep in mind that these are other people and your body is different from everybody.
l recommend you to make an appointment with a board certified plastic surgeon to talk about your goals and anything you need to know.
Best of lucks.
Dr. Luis Mejia