I like the idea and I would use apps like this in my own personal life, but as a UX designer this is also a super horrible experience if you don't give any affordance in the UI to the user indicating what type of action they can take.
I agree, that's one of the challenges we have to tackle.
The commands being contextual to the page you are on helps in my point of view, but we have to explore ways of onboarding users in a proper way.
Yes we've thought about that too. That's why we added autocompletion and we will add an interactive tutorial should we decide to commit to this.
Not just auto completion but interactive auto completion coming from a list of options. This is what I did for one of my projects - you get a vertical list of all possible inputs (within reason of course) which is narrowed down as you type.
You could use the arrows to navigate if you're not inclined to type - arrows - tab - arrows - tab would build a complete command sequence.
This is exactly what we did.
The gold standard of keyboard discoverability and low cognitive load has to be Microsoft office suite. The Alt key reveals all the shortcuts on screen, and as you type them the context changes to the sub menu you've selected. It's an astonishingly effective and useful accessibility feature. I wish this WAS the standard.
There's nothing better than for a user, especially one who relies on visuals, to visually see a "tree" of the possible actions they can do. And not tree in a literal or CS sense, but rather, buckets of activities with sub activities, not in a hierarchy but in an intuitive and context-creating way.