What can be done after thigh lipo, for loose, sagging thick skin?

I would suggest scheduling another appointment with your plastic surgeon.  Maybe you lost a lot of weight and might be a candidate for a thigh lift.  It is difficult to suggest anything further without photos or a consultation but best of luck! 

Without photos there is no way to give an accurate opinion.  Sometimes surgery is needed to remove the skin, or laser or radiofrequency tightening devices can be used.  It all depends on the extent of the laxity.  Seek an experts opinion or return to your physician for advice.  I wish you the best of luck, Dr. Emer.

Without photos I can only speculate but it sounds like you may benefit from a Lower Body Lift.  If the only problem is loose skin and the fat was removed evenly with no residual fat to remove then a Lower Body Lift should be successful in pulling up all the excess skin, tightening the thighs and buttocks and leaving a scar that is designed to be hidden by your panties, unlike the older, more traditional thigh lifts that placed the scar below the buttock where it can be seen.  Because all of the tension is held with permanent sutures in the deep tissues the scar usually heals very nicely.  A full Lower Body Lift would also tighten your tummy but if you don't need that then the incision can be brought down into the groin area to tighten the inside of your thighs if necessary.  A consultation with a plastic surgeon certified by The American Board of Plastic Surgery is the best way to explore your options.