It seems like a hard design problem because for some roles that Decker is trying to fill, it makes sense to be a standalone tool, but for others (especially as a prototyping tool), there's a benefit in connecting to the outside world.
Often when I'm building something, I think Decker would make a good piece of scaffolding. It feels like it ought to be easy to hook decker up to a live SQLite database and make a little frontend, or to design a contraption that could become a user interface component in a web page (outside of a deck).
Decker provides this really amazing development experience, but the price is that you have to be willing to make something that looks like a deck. Most of the time I'm making web pages that I want to look a certain way, and there's no clear path to go from one to the other. Once in a while, though, I have something that would work fine as a deck... And then the question is whether decker would let me make that thing.
I think having some reference docs that showed how to extend Decker would let power users do whatever they want, and then you could wait and see what people actually build.