A contribution to the spectrum of meaning that you refer to is Pipe dream = an idea or plan that is impossible or very unlikely to happen

Cambridge dictionary

That brings to mind pie in the sky, castles in the sky, fool’s paradise, or even a

chimera = a hope or dream that is very unlikely ever to come true

Cambridge dictionary

A contribution to the spectrum of meaning that you refer to is Pipe dream.

Pipe dream (noun): an idea or plan that is impossible or very unlikely to happen.

Example: Her plans are not realistic - they'll never be more than a pipe dream.

[Cambridge English Dictionary]

That brings to mind pie in the sky, castles in the sky, fool’s paradise, or even a chimera.

Chimera: a hope or dream that is very unlikely ever to come true.

Example: Why do people chase the chimera of fame?

[Cambridge English Dictionary]