Given a perfect runtime, what would change?

[xificurC]

Right now missionary works on the JVM and in JS. Given a custom ideal runtime

• what features would help make missionary simpler/better?
• what features would become obsolete or moved into the runtime directly?

I e.g. remember hearing multi-shot continuations would help in some way? I'm wondering which parts of the API are constrained by the current runtimes, which parts of the API are band-aids over missing runtime capabilities etc.