I would say general purpose libraries that can be used by other developers. A good example might be, for example, a better touchpad driver to match the Macbook

I would say general purpose libraries that can be used by other developers. A good example might be, for example, a better touchpad driver to match the Macbook one, as someone recently mentioned. Or for me personally: a macOS Sidecar clone. Or an easy to use, cross-distro way to run Windows apps inside a VM with GPU acceleration.