What can I do to improve my jawline? (photos)

I believe the best option is the combination of a few modalities. With age, we have redundant skin in the jowl area that has descended from the cheek area. Placing some cheek fillers in the mid/upper face can oftentimes displace quite a bit of that redundant skin from the jowl and jawline and lift it back out of the way. You would need approximately two syringes in your cheeks. I would recommend either Voluma, Lyft, or Radiesse.
Aside from that, I agree with the thought of adding some chin filler and even jawline filler. You would need approximately three syringes, one for each side of the jaw and one for the chin. The best filler for this area is Radiesse, as it is a calcium-based filler which mimics bone and is a biostimulant of collagen (meaning it stimulates your body to create its own collagen). This gives firm and long-lasting effects, which oftentimes last well over a year.
Lastly, you may be a candidate for Kybella in the under-chin area. Kybella is a fat-dissolving agent that we oftentimes use especially for this area, the submental fat. This dissolves fat and causes some sclerosis in the area which can contract the loose fatty tissue under your chin. Kybella is injected about once per month until the desired results are achieved. You would require approximately 3 sessions of Kybella for a significant result in this area.
Threads are not a bad idea to add at the very end for some fine adjusting and tweaking.

For unique cases where a variety of modalities could be used to treat the condition, I would suggest an in-person or virtual consultation to develop a custom and effective treatment plan. It is difficult to precisely define the best option without collecting the same information. I suggest reaching out to an expert for a consultation with surgical and non-surgical methods to get a comprehensive option. Best, Dr. Emer.

If you are looking for a quick, simple and temporary solution, fillers would make a very logical solution. A lifting filler placed in the upper outer cheeks would create a more heart-shaped, feminine shape that would also likely improve the under eye regions and smile lines. A volumizing filler placed in the region of the angle of the jaw and along the jawline would straighten not only diminish the jowl along the jawline, but would likely improve the upper neck and under chin regions and to some extent the marionette lines. Volumizing filler in the chin would not only help to harmonize the projections of the nose and the lips in profile, but would additionally likely improve the upper neck/under chin area. If still necessary (and it may not be) additional, less robust fillers could be used for directly addressing the infraorbital tear troughs and bags and for straightening the downturned angles of the mouth. I routinely perform all of these procedures in one sitting, if the patient wishes, in about a 45 minute session. The results usually engender an immediate "Wow!" reaction and give rise to little to no significant downtime. It would not be surprising that you would be so delighted with the results of the aforementioned and with the resulting secondary improvement in the upper neck and under chin regions that no further work would be deemed necessary. You would be wise to consult with an experienced, board certified cosmetic dermatologist and make sure to ask to see his/her personal befores and afters prior to agreeing to proceed. Best of luck