My current client needed to get requirements for a new system from people who do not think in terms of functional requirements, use cases, or even user stories. Their understanding of the future hypothetical system was too vague and undefined for those levels of detail.
Instead, I decided to run a session of Remember the Future (RtF). Not only does RtF encourage vision creation, it is also inexpensive and simple to run. The client understood that RtF generates material that then needs to be collated and translated into more "usable" requirements.
The session had good participation from marketing communication, business, and front-line customer interaction folk. The client was surprised by the amount of material that was generated, with about a third of it being directly translatable into requirements.
One of the reasons why Remember the Future is good for this sort of situation where specific detailed requirements are problematic is that by looking back from the successful "reality" of the future and telling us in the past what it is like up there, it leverages the human mind's ability to confabulate.
While confabulation is distressing from a politician explaining their fiascos and amusing in a four-year-old explaining how the dog got on the roof, it forces a pure "blue sky" vision into specifics and tangible details.
We can then derive requirements from the specifics and go back to the participants to check against the "reality" that they imagined.
The material that was not applicable to requirements is also useful because it allows us to set and communicate system boundaries.