There is https://gitlab.com/Rich-Harris/buble which is "smaller" but I don't think it's use case is the same. There is almost no configuration (not a bad thing at all) so no custom plugins, and it only compiles performant ES5. It's scope has expanded a bit (used to only be ES6, but now JSX, and people will continue to ask for more), and tries to compile to readable ES5 rather than be spec compliant.
However like any open source project you need a lot of maintainers to continually update it over the months/years. If someone wants to make something new instead, that sounds fine but depends on the goals and why you wouldn't contribute back to Babel already or refactor internally. No, I don't really see it happening, it's already hard enough to maintain Babel as is 😛