It could even be an opt-in feature, allowing you to execute mix deps.compile --parallel, and as we find cases that cause problems, we can tackle them. After some time, it could become the default with a flag to disable.
Looks very promising!
Or compile_parallel: false to disable, or as I mentioned, allow package authors to specify if it supports being compiled in parallel with others. Additionally, like extra_applications, you could specify a allow/block-list to override parallel compiler compatibility.