I would love to see official documentation and/or support for pnpm on CircleCI.
There is intention of adding a guide on the pnpm side (https://github.com/pnpm/pnpm/issues/1648) but I feel it'd be better for CircleCI to officially support it rather than having a third-party come up with that.
And with pnpm gaining traction–specially with developers working on monorepos–over npm and yarn (because of its performance) I feel it'd be the smart move.