Similar to Docker executors (https://circleci.com/docs/2.0/configuration-reference/#resource_class) it would be nice to have some CPU/RAM options for MacOS builders as well. One persistent drag on iOS is that builds are pretty slow, and while the hope is that Xcode 10 provides for faster builds (that's the hype anyways), people might also be willing to pay to throw better hardware at the problem and speed up the build that way. On the other end of the spectrum, MacOS workflows may have some jobs that could do just fine with a VM that is smaller than what is currently provided. Overall, it would be nice to be able to optimize a MacOS workflow by putting the right amount of compute resources right where they're needed, similar to how we can do it for Docker builders.
This doesn't seem totally unprecedented among MacOS CI providers - Bitrise for example provides a higher class of MacOS VM (for an increased expense of course): https://www.bitrise.io/pricing