AVIF is a new image format (based on the open AV1 video format) that has the potential to produce significantly smaller file sizes than existing formats, although with different tradeoffs. I find demos of the format to be quite compelling: https://jakearchibald.com/2020/avif-has-landed/

I realize that webp support is probably higher priority, since caniuse.com has webp at 85% support while avif is currently at 25%, but I think it is likely to gain support quickly, since Firefox already has a decent implementation (behind a flag) and Apple are a member of the group that created AV1.

UPDATE August 2021: caniuse.com now has avif at 67.25% support. The next version of Firefox will have it enabled by default, it seems.