JPEG-XL (someone already mentioned it as .jxl below) image files.
competitive with AVIF compression levels
not recycling video compression, so you get benefits like progressive loading
JPEG transcoding - can take existing JPEG files (so much of the existing images online) and shrink their size by ~20% with literally no change to the presented image, and this is easily reverable. The amount of data this would shrink without risk of altering the data is HUGE.
JPEG-XL (someone already mentioned it as .jxl below) image files.
There are a ton of other benefits but those are the three I’m most excited about.