TL;DR
A key combination, unbound by default, that closes and restarts the game, like it does when you change your mod settings.Either it could be in the super secret settings, or probably just unbound by default in the default controls menu.
Why?
I'm currently developing a mod and I spend an awful lot of time closing Factorio, then tabbing back over to Steam among my million windows and launching the game again. Having a shortcut for this would save me a lot of time overall. Hopefully it would be easy to implement (the game already has infrastructure to restart the game, like when you fiddle with mod settings). It's a small annoyance but it would be one rough corner of modding shaved off