Depends on the size you want to tackle. Let's shoot for MIT licensed or similar, so we don't have to do the unethical thing.

ESLint or Webpack would probably be attempts that are decently sized for a challenge.

Cheerio would be a bit smaller.

Chalk is probably close to the absolute lower bound of what's even meaningful. (You'd likely just regenerate a package that size from scratch instead of wondering about compat)