My stomach is dented after lipo over a year ago. What can be done? (photos)

The outcome of a liposuction revision depends on what the circumstances were in the first place.  I see no stretch marks on your abdomen which indicates a better chance of a good revision.  If you had classic liposuction with a canula and the muscle in the surgeon's arm, then the chance of excellent revisionary surgery is much greater.  On the other hand, if you had Vaser, Laser, Slim, Smart or all those other silly names, your surgeon may have cooked your connective tissue leading to much less chance of a good revision surgery outcome.Ultrasound (the kind physical therapists use for sore muscles) after classic liposuction can break up some of the scar tissue, improving the smoothness of your post liposuction skin.  This has some value at one year post op, but much less than if it was done immediately after surgery.  A second procedure one year after with wavy dented skin should consist of the use of a blank canula with no tunnel to remove fat and minor fat grafting to introduce intact fat cells to fill out the area between the skin and underlying scar tissue.  Immediately after the second surgery is done, ultrasound can act like a sifter in the kitchen to smooth the fat cells down into the deeper scar crevices.  Also, the use of DMSO after revisionary surgery can increase the number of blood vessels present to help support the survival of the newly grafted fat cells.  Skin tightening may be useful if you have had one or more children or have lost significant weight.  However, your pictures do not indicate much loose skin, and therefore I doubt this would be of much value.

An examination would be necessary to determine an appropriate surgical plan: more liposuction, fat grafting, or both. Things that have no effect are internal or external heating from any energy source, be it light, sound, or radiofrequency. Defects are due to irregular thickness of the fatty layer and/or scarring. Techniques developed and propagated by Simion Wall, Jr in Shreveport offer the best known way to smooth liposuction irregularities. It is not a special machine, but a protocol of techniques using various liposuction cannulas, and is known as SAFElipo. Since it is technique driven, it requires experience to achieve the best result possible. 

this will be VERY dificult tot fix it may even require a mini tuck, fat grafting or internal heating. please see an expert as over 60% of my practice is now fixing these types of procedures. 
You need a formal evaluation by an expert in body shaping and contouring to determine what is best.  Traditional liposuction awake and smart liposuction are old methods and do very poorly for tightening skin or significant skin tightening or contouring.  Small problem areas can be improved slightly by these methods.  The best in body contouring not only removes fat with energy devices like VASER ultrasound but also internally heat the skin with radio-frequency devices such as J PLASMA/RENUVION, InMode BodyTite, ThermiRF to heat the under surface of the skin and tighten it at the same time as fat removal. These combinations are called 4D HIDEF VASER contouring and can only be done by a well . trained body contouring specialist. At the same time fat should be harvested, PRP and stem cells added and injected into muscles or indentations/cellulite to get body shaping and contouring.  This is known as a Brazilian butt lift when you inject the fat to the buttock for contouring and lifting, but fat can also be used in areas such as the chest/breast, arms/shoulders, calves and face.  Cellulite should also be treated with cellfina, sculptra, subcision and/or fillers such as bellafill, renuva synthetic fat, or sculptra.  If skin is very loose before surgery then it will need to be removed such as lower abdominal skin (tummy tuck/abdominoplasty), breast skin (breast lift), arm skin (brachioplasty).  Overall you need a full assessment, but simple liposuction alone will not tighten skin and needs a combination of various methods -- fat removal, skin tightening, defining, fat grafting, skin removal -- to get the best in body contouring and body sculpting.  Dr. Emer.