> A command-line shell at which you can type pipelines, involving some regular CLI commands, but also GUI commands as components, and when the pipeline is run, those GUIs will pop up in the middle of the pipeline, allow you to interact with them, and then any data output from them will go to the next component in the pipeline

There seems to be some precedent for this sort of thing. For example, DVTM can invoke a text editor as a "filter", where the editor UI is drawn on stderr and result saved to stdout.