I find it interesting that the OP has had essentially zero feedback on anything other than confirmation dialog boxes - especially since I don't think that was part of the OPs speculation.
Having some form of 'interactive spec' where stakeholders can 'play with' the system to verify intended behaviour is a really interesting idea. Of course it is used in software development all the time to varying levels of fidelity (wireframes, mockups, interactive prototypes and so on). But I think here is the rub... the better the quality the simulation the greater the amount of effort until it becomes approximately equal to the cost of just doing it.
There is no doubt in my mind that streams will be a much bigger part of the future than they are now. (They won't completely replace page-based content where that is more appropriate.) Currently streams seem to be the domain of the big 'platform' players (Twitter, Facebook, Instagram, maybe a feedreader for those so inclined) so I am only really have to 'keep tabs' on a couple of streams.
I think our current tools for creating and consuming streams are still relatively basic and for 'streams' to break free from these strongholds requires a lot more innovation in how we can create them, interact with them, manipulate them and transition in and of of them seamlessly.
Having some form of 'interactive spec' where stakeholders can 'play with' the system to verify intended behaviour is a really interesting idea. Of course it is used in software development all the time to varying levels of fidelity (wireframes, mockups, interactive prototypes and so on). But I think here is the rub... the better the quality the simulation the greater the amount of effort until it becomes approximately equal to the cost of just doing it.
Maybe with enough automation and tooling...