This. I'm SUPER excited to have a Swift backend for a web app, and/or I've been toying around with the core logic of another app. Either idea doesn't need anything in the way of GUI, just the standard library and maybe a few things from Foundation that should be in the standard library anyway (like pseudorandom number generation).
"Essential" ? Any of the objects that can give a "dashboard" interface and maybe draw a picture.
I'd love a portable GUI API that satisfies some harsh conditions:
1) Portable between mac,linux,windows,android,ios. [ must re-compile to target ]
2) Compilable into a single static binary.
3) New developments don't break backwards compatibility.