> My first thought is a package system based on melpa / use-package from emacs.
That would be great! Use-package is amazing, and that (combined with similar loading things from package.el) could make loading dependencies way easier. I know that when I'm writing arc code, I'm reluctant to use libraries -- even libraries included with arc -- because they're (afaik) impossible to automatically load.
> We could also experiment with some avant garde packaging ideas, such as akkartik's thoughts on avoiding version pinning...
Amusingly, the Emacs ecosystem can be thought of as a package manager that avoids version pinning; the standard package repositories (GNU Elpa, Melpa) only keep the latest version of a package around; you can't install older versions.
Of course, I know of no package foo.el that introduced _foo2.el_ when breaking backwards compatibility. So we can maybe do better that way.