The first obvious risk that comes to mind is that nothing can be made that correctly takes into account everything it might some day be required to be able perform, so it can easily be done wrong and would probably need to be very flexible / loosely coupled.

And while the distributed aspect adds complexity, I feel that involving different devices, not all of which will on, or online, all the time, makes it a necessity. Not accounting for that would doom it.

But yes, that's a big part of it.

For the interaction / capabilities discovery part, I've lately been drawn to some sort of declarative interface, akin to Apple's "App Intents", dynamically exposed depending on the system's state. It also reminds me of how REST APIs were supposed to be discoverable.

But I'm not sure, and it could be a dead-end.

Another thing that comes to mind is Bell's Plan 9, and how someone who used to work there said that when he would come home, he'd just open his computer and everything would be there, just like he had left it at work. It's not enough, and the goal wouldn't be to have the exact same interface replicated everywhere, but this single distributed state, with each device just being a window into it, feels like a start.

Not that "it" would be an operating system. That too would be a doomed effort (many people can't change their OS nowadays, and most people who could wouldn't do it just to use a product or service). It would have to a paradigm, and perhaps a framework, or a language.

You've got me thinking again. I'm not going to be able to get anything done for days now.