Parallel and sequential, especially at the command level, are really the wrong abstractions for running scripts. If you have multiple packages, each with builds, there's a high chance you have dependencies and multiple packages depending on common ones. What you really want is a way for scripts to describe their dependencies, and then the runner figures out what order to run them in, and cache scripts that don't need…
DAG + content-addressing, final binary being the target and everything resolved from there. We could have some beautiful build system that just works and is fast, but seems it never magically appears by itself although it seems so elegant. Guess Nix/NixOS is the closest we've gotten so far, works well enough, missing concurrency and parallelism though.