There are plenty of efforts to improve JPEG compression while retaining compatibility with existing decoders. But there's only so much you can do without breaking decoder compatibility, and if you do that then that's a new format whether you call it JPEG or not. It would be super confusing to have .jpg files that wouldn't work in existing software.
JPEG XL is a new format with a new file extension jxl, but it does have backwards compatibility features that look pretty nice. Maybe that's what you want.
mozjpeg greatly improved the state of art jpeg encodig, while maintaining full backwards compatibility https://github.com/mozilla/mozjpeg
but there's a limit on what can be done with the primitives that jpeg offers. for example, jpeg is stuck with the older huffman coding for the entropy encoding part, instead of the better arithmetic coding or asymetric numeral systems
