Last week, as part of a visioning/future planning exercise for local education, I ran a Future Backwards exercise with Year 4 - a group of 8-9 year-olds. It reinforced the power of the exercise - but also that if you create the conditions for discussion, allow time for that to happen and wait for things to emerge, they will.
To the horror of some of the adults around (a visiting party of headteachers who couldn't help themselves but interrupt and offer inappropriate judgments until they were herded away), initial thoughts ranged from "a big swimming pool for the boys and a little one for the girls - and the boys one is filled with jelly" to "lady teachers with big boobies". After a while it moved through "but that's too much play - we want to have some lessons" onto real, useful thoughts and concepts.
Much the same concern often crops up in other organisations, particularly those with faintly (or strongly) paternalistic views of their staff. "We can't let them just talk about anything, they'll get so negative/unrealistic/etc"
Patience, a simple setup and permission to be negative/unrealistic will, in short order, create useful emergent ideas.