Already supported via the 18.20 preview release:
1. Ubuntu 18.04 (and all distros) include a full open source driver stack
2. As fermulator suggested, the open source stack is recommended for gaming and general use - the amdgpu-pro option is really aimed at CAD and other workstation applications.
3. The 18.10 and 18.20 driver releases on amd.com include both all-open-source and -PRO options, so if you want the very latest open source drivers you can download them and install without the -pro option. If you need OpenCL but want to stay with the all-open stack otherwise you can install that combination, although AFAIK the OpenCL/OpenGL interop functions only work with the -PRO GL driver.